Austenitic cast iron and manufacturing process for the same, austenitic-cast-iron cast product and component part for exhaust system

ABSTRACT

An austenitic cast iron according to the present invention has Ni: from 7 to 15% by mass, and is characterized in that it comprises a base structure in which an austenite phase makes a major phase even in ordinary-temperature region by adjusting the respective compositions of Cr, Ni and Cu, excepting C and Si, so as to fall within predetermined ranges. In accordance with the present invention, it is possible to obtain an austenitic cast iron, which is excellent in terms of oxidation resistance and the like, inexpensively, while reducing the content of expensive Ni.

TECHNICAL FIELD

The present invention relates to an austenitic cast iron, which isexcellent in terms of heat resistance, and the like; to a cast product,which is comprised of that; to a production process for the same; and toa component part for exhaust system.

BACKGROUND ART

It is often the case that members being formed as complicatedconfigurations, and relatively large-size members are manufacturedbymeans of casting; what is more cast products being made of relativelyinexpensive cast irons (hereinafter referred to as “cast products”simply) are used frequently.

In cast iron, C in the alloy whose major component is made ofiron-carbon exceeds the maximum solid solubility limit in γ iron (e.g.,about 2% by mass), and the cast iron is accompanied by eutectoidsolidification. Usually, in order to improve the characteristics, suchas the mechanical properties, corrosion resistance and heat resistance,various alloying elements are added. Such a cast alloy is referred to asan alloy cast iron, and especially those cast irons with greatalloying-element contents are referred to as high-alloy cast irons.These high-alloy cast irons are usually divided into ferritic cast ionsand austenitic cast irons roughly depending on the difference betweenthe crystalline structures of their crystallizing bases.

Among them, since the austenitic cast irons are comprised of austenitephase (or γ phase) mainly, not to mention in high-temperature region,but in ordinary-temperature range as well, they are good in terms ofheat resistance, oxidation resistance, corrosion resistance, and thelike; and are moreover good in terms of ductility, toughness, and soforth.

Accordingly, the austenitic cast irons are often used for members thatare made use of in harsh environments such as high-temperatureatmospheres. For example, speaking of the field of automobiles,turbocharger housings, exhaust manifolds, catalyst cases, and the like,are given. Any one of the members is a component part, and the like,which is exposed to high-temperature exhaust gases, and consequentlywhich is required to exhibit long-term durability.

By the way, various types are available in the austenitic cast irons aswell, and the following are representative ones: Niresist, nimol,nicrosilal, monel, minober, nomag, and the like. Moreover, in JapaneseIndustrial Standards (i.e., JIS), too, 9 types are prescribed for theflake graphitic cast iron (e.g., FCA), and 14 types are prescribed forthe spheroidal graphitic cast iron (e.g., FCDA).

In the conventional austenitic cast irons, an austenite phase has beenmade obtainable even in ordinary-temperature range by having themcontain Ni, namely, an austenite stabilizing element, in a large amount(Ni: from 18 to 36%, for instance). This Ni is expensive considerablycompared with Fe, namely, the parent material, and the other alloyingelements, and consequently cast products comprising the conventionalaustenitic cast irons have been high costs considerably.

Surely, like Niresist (FCDA-NiMn137 as per JIS), an austenitic cast ironwhose Ni content is less comparatively has also come to be knownpublicly. However, Niresist (FCDA-NiMn137 as per JIS) is poor in term ofoxidation resistance. Moreover, when observing Niresist by X-rayanalysis (or XRD), the austenite proportion becomes 100%. In actuality,however, it takes a lamellar structure (i.e., a structural constructionin which a plurality of long and thin rod-shaped constructions are linedup so that a striped pattern is seen) in which lamellar carbides exist,in addition to the austenite structure, in the Fe base, as can beunderstood when looking at photographs in FIG. 5 (that is, the twophotographs on the left side). Therefore, Niresist has such a structurethat it is not possible to say any longer that the austenite phase makesa single phase.

Incidentally, when lamellar (or acicular) carbides exist in addition tothe austenite structure, tensile stress occurs in austenite upon beingheated, because those carbides whose thermal expansion coefficient isgreater than that of the austenite expand more than the austenite does.Accordingly, in the case where Niresist (FCDA-NiMn137 as per JIS) isused for members, like automotive exhaust-system components part thatare exposed to high temperatures and ordinary temperature repeatedly,there is such a fear that cracks might occur in the austenite structurebecause tensile stress occurs repeatedly in the austenite structure.Further, austenite phase has a great solute carbon content compared withthat of ferrite phase. Accordingly, upon the transformation from ferriteto austenite, ambient graphite comes to be dissolved/solid solvedtherein as being accompanied by the austenitizing; then, it becomeslikely to make voids in the graphite sections; and then the degradationof the resulting cast product's strength is facilitated. Furthermore,chilling, which results from supersaturated C, is facilitated because ofreturning back to ferrite phase again at the time of cooling;consequently, the following are feared: the increase of chill phasebeing accompanied by cooling/heating cycle; embrittlement beingaccompanied therewith; and volumetric expansion.

Moreover, when being an unstable structural construction in whichlamellar carbides, and the like, exist in addition to the austenitestructure, the resulting austenitic cast irons have such a drawback aswell that the workability worsens, because work-induced martensite thatis very hard appears at the time of cutting work.

In addition, an austenitic cast iron whose Si content is increased whilemaking the Ni content much less than that of aforementioned Niresist isdisclosed in a patent literature mentioned below.

Patent Literature No. 1: Japanese Unexamined Patent Publication (KOKAI)Gazette No. 58-27,951

DISCLOSURE OF THE INVENTION Assignment to be Solved by the Invention

Aforementioned Patent Literature No. 1 discloses that, in relation tooxidation resistance, namely, an index of heat resistance concerningaustenitic cast iron, the more the Si content enlarges the less theoxidized weight increment per unit area becomes (see FIG. 6 of PatentLiterature No. 1). However, according to studies by the presentinventors, the Si content that becomes excessive results in decliningthe elongation of austenitic cast iron, and in worsening themachinability. Consequently, considering the reliability and themass-producibility of heat-resistant members comprising austenitic castirons, and the like, it is not practical to enhance the oxidationresistance to such a level that is sufficient in view of practical useby only adjusting the Si content.

The present invention is one which has been done in view of suchcircumstances. Specifically, it is an object to provide a low-costaustenitic cast iron that is an austenitic cast iron whose containedamount of Ni is less, and which is excellent not only in terms ofthermal-fatigue strength, and the like, but also in terms of oxidationresistance. Moreover, in addition to that, it is another object toprovide austenitic cast products comprising that austenitic cast iron,and a manufacturing process for the same, and furthermore exhaust-systemcomponent parts, namely, some of those austenitic cast products.

Means for Solving the Assignment

The present inventors studied earnestly to solve this assignment; as aresult of their repeated trial and error, they succeeded in obtaining anaustenitic cast iron that exhibited favorable characteristics, even inthe case of reducing the contained amount of nickel (Ni), by adjustingthe contained amounts of carbon (C), silicon (Si), chromium (Cr), nickel(Ni), manganese (Mn) and copper (Cu). In particular, it was possible toobtain an austenitic cast iron that exhibited good oxidation resistanceby adjusting the Cr content and/or the Cu content without everincreasing the Si content excessively, even while reducing the Nicontent. The present inventors arrived at completing a variety ofinventions, which will be described later, by developing theseachievements.

(Austenitic Cast Iron)

(1) Specifically, an austenitic cast iron according to the presentinvention is characterized in that:

it comprises:

basic elements comprising carbon (C), silicon (Si), chromium (Cr),nickel (Ni), manganese (Mn) and copper (Cu); and

the balance comprising iron (Fe), inevitable impurities and/or atrace-amount modifier element, which is effective in improvingcharacteristic, in a trace amount;

it is an austenitic cast iron being a cast iron that is structured by abase comprising an Fe alloy in which an austenite phase makes a majorphase in ordinary-temperature region;

wherein said basic elements fall within compositional ranges thatsatisfy the following conditions when the entirety of said cast iron istaken as 100% by mass (hereinafter being simply expressed as “%”):

C: from 1 to 5%;

Si: from 2 to 6%;

Ni: from 7 to 15%;

Mn: from 0.1 to 8%;

Cu: 2.5% or less;

Cr: 6% or less; and

Cr+Cu: 0.5% or more.

(2) First of all, the Ni content becomes a considerably small amountrelative to the entire cast iron in the austenitic cast iron accordingto the present invention. In view of the conventional technical commonsense, it seems that no base is obtainable, base in which an austenitephase, which is stabilized in ordinary-temperature range, makes a majorphase. However, in the present invention, an austenite phase wasobtained successfully by setting, even though on the premise of thatsmall-amount Ni content, the respective contained amounts of the otheralloying elements, namely, C (especially, C_(s), a solute carboncontent), Si, Cr, Mn and Cu to appropriate ranges that satisfy theaforementioned respective conditions.

In particular, in the austenitic cast iron according to the presentinvention, the oxidation resistance, which is indexed by alater-described oxidized weight decrement, and the like, is improved bymeans of containing Cr or Cu in an adequate amount even whilesuppressing the upper limit of the Si content.

It is believed herein that Cr forms a passive film, which comprisesdense and fine chromium oxides, adjacent to the surface of theaustenitic cast iron and has then improved its oxidation resistance.Moreover, Cr combines with carbon in the cast-iron base to precipitatecarbides therein, and accordingly is capable of improving thehigh-temperature proof stress of the cast iron by means of precipitationstrengthening of the base. However, Cr that becomes excessive is notpreferable, because carbides increase so that the toughness andworkability, which are indexed by means of the Charpy-impact value andso forth decline. Hence, in the austenitic cast iron according to thepresent invention, it is preferable that the Cr content can be from 0.5to 4%.

Further, when Cr is contained, such an effect is available that thestructure is stabilized so that lamellar and/or acicular carbides areless likely to emerge, as can be also understood from the table of Gibbsfree energies.

Moreover, Cu yields an effect of making the fcc structure more stable,because it has an fcc structure, namely, a crystalline structure that issimilar to austenite at ordinary temperature, and because it has a densestructure that is much less likely to pass oxygen than is ferrite with abcc structure. And, Cu does not at all enter oxidized film, and then Cuis enriched at the interface between the oxidized film and metal;accordingly Cu turns into an fcc structure possessing the latticeconstant that differs from that of parent phase; consequently Cudemonstrates a barrier-layer effect that inhibits the interstitialaction of oxygen atoms possessing such an energy state that they arelikely to force into the parent base; and it is believed therefore tohave its oxidation resistance improve.

Moreover, in addition to stabilizing an austenite structure by solvingin the base in the same manner as Ni does, Cu is an effective elementfor refining crystalline particles in the base's structure and thenhaving the high-temperature proof stress improve. Further, as a resultof being studied earnestly by the present inventors, it was understoodthat Cu also yields an effect of decreasing hardness, and consequentlyit is possible to intend to improve the workability of austenitic castproduct.

However, when Cu becomes excessive, the peritectic structure of Cuemerges to hinder graphite's spheroidizing; consequently, the strengthand the like of cast iron decline, or the peritectic structure of Cuemerges so that elongation performance worsens at the time of hightemperatures. Therefore, it is preferable to contain Cu within such arange that does not worsen ductility at the time of high temperatures.Hence, it is allowable to set the upper limit of Cu at 2.5%, forinstance.

In the present invention, since to have oxidation resistance improve isone of the objectives, it is preferable to involve Cu and Cr, whichimprove oxidation resistance, in an amount of 0.5% or more by sum total.It is preferable that the lower limit of this Cu+Cu can be 1%, 1.5%, orfurther 2%.

And, it is suitable to set the Ni content to from 8 to 14% in obtainingan austenitic cast iron, which is provided with strength, heatresistance (including oxidation resistance), elongation, ductility,toughness, workability, and the like, in a well balanced manner like thepresent invention, at low cost.

(Austenitic Cast Product and Manufacturing Process for the Same)

(1) It is possible to grasp the present invention not only as theabove-described austenitic cast iron but also as an austenitic cast ironthat comprises that austenitic cast iron. As some of the examples of anaustenitic cast product according to the present invention, it ispossible to give members, such as exhaust-system component parts, whichare exposed in high-temperature environments.

(2) Further, it is possible to grasp the present invention as amanufacturing process for that austenitic cast product as well.

Specifically, it is permissible that the present invention can even be amanufacturing process for austenitic cast iron that is characterized inthat it comprises:

a molten-metal preparation step of preparing a molten metal with theaforementioned compositional range;

a pouring step of pouring the molten metal into a casting die; and

a solidification step of cooling the molten metal that has been pouredinto the casting die, and then solidifying the molten metal;

wherein a cast product comprising the above-described austenitic castiron according to the present invention is obtainable.

(3) By the way, in expanding applications of the austenitic cast iron(or cast product) according to the present invention, it is also oftenthe case to add various modifier elements at the time of casting. Forexample, it is often the case that an auxiliary agent is added in orderto increase the number of graphite particles that crystallize instructures of the base, or in order to spheroidize their configurations.

Hence, it is permissible that the manufacturing process for austeniticcast iron according to the present invention can even be one beingcharacterized in that it comprises:

a modifier-free-molten-metal preparation step of preparing amodifier-free molten metal comprising a molten metal with thecompositional range as set forth in either one of claims 1 through 15;

an auxiliary-agent addition step of adding an auxiliary agent, whichincludes at least one member being selected from the group consisting ofinoculant agents that make cores of graphite to be crystallized orprecipitated, and spheroidizing agents that facilitates spheroidizing ofthe graphite, to the modifier-free molten metal directly or indirectly;

a pouring step of pouring a molten metal into a casting die, the moltenmetal being after the auxiliary-agent addition step or during theauxiliary-agent addition step; and

a solidification step of cooling the molten metal that has been pouredinto the casting die, and then solidifying the molten metal;

wherein a cast product comprising the aforesaid austenitic cast iron isobtainable, the austenitic cast iron in which substantially spheroidalgraphite is crystallized or precipitated within the resulting base.

(Additional Constitution)

It is allowable that the austenitic cast iron (including the austeniticcast product) according to the present invention, or the manufacturingprocess for the same according to the present invention, can havecontents as set forth below. Moreover, it is even allowable to furtheradd one or two or more constitutions, which are selected arbitrarilyfrom the constitutions that are listed below, to the aforementionedpresent invention.

Note that is should be notified that it is feasible to add theconstitutions, which are selected from those set forth below,additionally to a plurality of inventions in a superimposed manner andarbitrarily. Moreover, it is feasible to combine any one of theconstitutions, which are set forth below, with each other appropriatelybeyond the categories. For example, it is needless to say that, whenbeing one of the constitutions that are directed to a composition of theaustenitic cast iron, it can be relevant to the austenitic cast iron aswell as the manufacturing process for the same. In addition, although itappears at first glance to be a constitution that is directed to a“process,” it can turn into a constitution that is directed to a“product” when comprehending it as a product-by-process.

Another austenitic cast iron according to the present invention ischaracterized in that it comprises:

basic elements comprising C, Si, Cr, Ni, Mn and Cu; and

the balance comprising Fe, inevitable impurities and/or a trace-amountmodifier element, which is effective in improving characteristic, in atrace amount;

it is an austenitic cast iron being a cast iron that is structured by abase comprising an Fe alloy in which an austenite phase makes a majorphase in ordinary-temperature region;

wherein: a carbon equivalent (hereinafter being simply expressed as“C_(eq)”) according to one of the following expressions being given bythe respective contained amounts of C and Si satisfies a first conditionaccording to the following expressions; and simultaneously the containedamounts of Ni satisfies a second condition according to the followingexpressions; and the contained amount of Cu satisfies a third conditionaccording to the following expressions; when the entirety of said castiron is taken as 100% by mass (hereinafter being simply expressed to as“%”); and

a nickel equivalent (hereinafter being simply expressed as “Ni_(eq)”)according to another one of the following expressions being given by therespective contained amounts of Ni, Mn and Cu and a solute carboncontent (C_(s)), namely, a content of C being solved in Fe, and achromium equivalent (hereinafter being simply expressed as “Cr_(eq)”)according to still another one of the following expressions being givenby the respective contained amounts of Cr and Si fall withincompositional ranges that satisfy fourth and fifth conditions accordingto the following expressions when the entirety of said base is taken as100%:

First Condition: 2≦C_(eq)≦5;

Second Condition: 7≦Ni≦15 (%);

Third Condition: 0≦Cu≦2.5 (%);

Fourth Condition: A₁·Cr_(eq)+B₁≦Ni_(eq)≦30 where “A₁”=−0.8, and“B₁”=21.6;

Fifth Condition: C_(eq)≦13.5;

Carbon Equivalent: C_(eq)=C+Si/3;

Nickel Equivalent: Ni_(eq)=Ni+30·C_(s)+0.5·Mn+Cu; and

Chromium Equivalent: Cr_(eq)=Cr+1.5Si

(2) In this austenitic cast iron according to the present invention, theNi content is set to a considerably small amount relative to the entirecast iron, as specified in the second condition. Consequently, in viewof the conventional technical common sense, it seems that no base withaustenite phase, which is stabilized in ordinary-temperature range, isobtainable.

However, in the present invention, an austenite phase was obtainedsuccessfully by setting, even though on the premise of that small-amountNi content, the respective contained amounts of the other alloyingelements, namely, C (especially, C_(s)), Si, Cr, Mn and Cu, to properranges that satisfy the aforementioned respective conditions.Hereinafter, the respective conditions that prescribe the presentinvention will be explained.

First of all, the carbon equivalent (C_(eq)) is prescribed like thefirst condition, because the present invention is anyway a cast iron,which is accompanied by peritectic solidification.

Next, the Ni content is prescribed like the second condition, becausethe present invention is a cast iron whose Ni is reduced. Even whenconsidering the second condition, relative to the composition of theentire cast iron, on the premise of the first condition, the austeniticcast iron according to the present invention is distinguishable frommany other conventional austenitic cast irons.

Further, in the present invention, the Cu content is prescribed like thethird condition in order to obtain an austenitic cast iron that isexcellent in terms of elongation performance at the time of hightemperatures. As a result of experiments done by the present inventors,it was found out that peritectic Cu exists in austenitic cast irons thatinclude Cu abundantly in the analyzed compositions. It is speculatedthat the resultant peritectic Cu worsens the elongation performance ofthe austenitic cast irons at the time of high temperatures.

Moreover, in the present invention, attentions are focused on basescomprising Fe alloys, on the basis of those Ni content and C_(eq). Thatis, such indexes as the nickel equivalent (Ni_(eq)) and chromiumequivalent (Cr_(eq)) that are found from the basic elements wereintroduced, and then a composition of the entire base, which makes thecore of the cast-iron structure, is determined by means of the fourthand fifth conditions.

As a result of experiments this time around, it was found out austeniticcast irons, which satisfy the aforementioned fourth and fifthconditions, do not have any lamellar structure that exists in austeniteeven when setting Cu to fall in the aforementioned range. And, it isspeculated that they are materials that are strong against thermalfatigue, because no lamellar structure exists in austenite.

These fourth and fifth conditions are based on the Schaeffler'sstructural diagram. As it is evident by referring to many famoustechnical literatures that the Schaeffler's structural diagramoriginally specifies the relationship between the composition and weldstructure that are relevant to welded section, or the relationshipbetween the composition and structure that are relevant to austeniticstainless cast steel or the like. That is, the Schaeffler's structuraldiagram is not necessarily used for cast-iron structure with greatcarbon content essentially. This issue is also understandable from thefact that the solute carbon content is used in the conversion ofNi_(eq).

Considering such circumstances, although it seems that theaforementioned fourth and fifth conditions are equivalent to theSchaeffler's structural diagram at first glance, it is possible to saythat the fourth and fifth conditions are totally novel conditionalformulas, which have been obtained from various sincere experiments doneby the present inventors, in the field of cast iron that differs fromsuch fields that the Schaeffler's structural diagram intends foressentially. Therefore, the austenitic cast iron whose basic elementssatisfy the aforementioned first through fifth conditions is anepoch-making cast iron that is not on the extension of the conventionaltechnical common sense.

Note that it is natural that the austenitic cast iron according to thepresent invention exhibits austenite cast iron's other excellentproperties because of its structure and composition.

(3) In the austenitic cast iron according to the present invention, itis possible to identify its composition, on the premise of the Nicontent being specified in said second condition, by setting therespective alloying elements, which constitute the basic elements,individually, or combining themvariously, with a plurality of methods,that is, other than the methods being prescribed as described above oralong with the methods being prescribed as described above.

For example, it is also permissible to set the lower limit of C_(eq) at2.1%, or further at 2.5%; and it is even permissible to set its upperlimit at 4.5%, or further at 4.3%, and the like. Moreover, it is alsopermissible to set the lower limit of Cat 2.1%, or further at 2.5%; andit is even permissible to set its upper limit at 4.5%, or further at4.3%. In addition, it is also permissible to set the lower limit of Siat 2%, or further at 3%; and it is even permissible to set its upperlimit at 6%, at 5%, or further at 4.5%, and so forth.

Moreover, it is also permissible to set the lower limit of Cr at 0%, at0.1%, at 0.2%, at 0.3%, at 0.5%, at 1%, or at 1.2%; and it is evenpermissible to set its upper limit at 9%, at 7%, at 5%, at 4%, at 3%, orat 2%, and the like.

It is also permissible to set the lower limit of Cu at 0%, at 0.1%, at0.2%, at 0.3%, at 0.5%, at 0.7%, or at 1%; and it is even permissible toset its upper limit at 2%, at 1.7%, at 1.5%, or at 1.3%, and the like.Note that, when the lower limit of Cu is referred to as 0% in thespecification of the present application, it not only means 0%≦Cu butalso 0%<Cu.

It is also permissible to set the lower limit of Mn at 3%, at 4%, or at5%; and it is even permissible to set its upper limit at 15%, at 10%, at9%, at 8%, or at 7%. Details on the actions and compositions of each ofthese elements will be described later.

Note that it is possible to use these upper limits and lower limitsindependently, or to combine them arbitrarily to use; and that it ispossible to set up ranges in which the upper limits make lower limits,or moreover to set up ranges in which the lower limits make upperlimits. Moreover, as far as it is possible to identify the compositionsof the basic elements, it is possible to combine compositions for therespective alloying elements arbitrarily to use. These notes are commonissues in the present specification.

On the basis of above, an example in which the austenitic cast ironaccording to the present invention is prescribed by a composition of theentire cast iron is specified as follows. Specifically, the presentinvention can be an austenitic cast iron that comprises: basic elementscomprising C, Si, Cr, Ni, Mn and Cu; and the balance comprising Fe,inevitable impurities and/or a trace-amount modifier element, which iseffective in improving characteristic, in a trace amount; and which is acast iron that is structured by a base comprising an Fe alloy in whichan austenite phase makes a major phase in ordinary-temperature region;and the present austenitic cast iron can be prescribed as an austeniticcast iron as well that is characterized in that:

said basic elements are set so that not only a carbon equivalent(hereinafter being simply expressed as “C_(eq)”) according to thefollowing expression that is given by means of the respective containedamounts of C and Si, but also Ni, Cu and Si fall within compositionalranges that are specified as mentioned below when the entirety of saidcast iron is taken as 100% by mass (hereinafter being simply expressedas “%”):

2≦C_(eq)≦5 (%) ;

7≦Ni≦15 (%);

0≦Cu≦2.5 (%); and

3≦Si≦6 (%)

Moreover, on the basis of above, another example in which the austeniticcast iron according to the present invention is prescribed by acomposition of the entire cast iron is specified as follows.Specifically, the present invention can be an austenitic cast iron thatcomprises: basic elements comprising C, Si, Cr, Ni, Mn and Cu; and thebalance comprising Fe, inevitable impurities and/or a trace-amountmodifier element, which is effective in improving characteristic, in atrace amount; and which is a cast iron that is structured by a basecomprising an Fe alloy in which an austenite phase makes a major phasein ordinary-temperature region; and the present austenitic cast iron canbe prescribed as an austenitic cast iron as well that is characterizedin that:

said basic elements are set so that not only a carbon equivalent(hereinafter being simply expressed as “C_(eq)”) according to thefollowing expression that is given by means of the respective containedamounts of C and Si, but also Ni, Cu and Cr fall within compositionalranges that are specified as mentioned below when the entirety of saidcast iron is taken as 100% by mass (hereinafter being simply expressedas “%”):

2≦C_(eq)≦5 (%);

0≦Ni≦15 (%);

0≦Cu≦2.5 (%) ; and

0.5≦Cr≦9 (%)

(4) By the way, it is not needed that the “austenite phase” beingreferred to in the present invention be an austenite single phasecompletely. That is, the clause, “austenite phase makes a major phase,”purports to make the following permissible: of course not only such acase as being comprised of an austenite single phase alone that exhibits100% austenite by X-ray analysis, and which does not include anylamellar structure that is made of those like martensite and perlite inthe austenite; in addition thereto but also such a case as including amartensite phase slightly.

If being notified daringly, it is allowable that an austenite singlephase can be present more than 50% by volume, 60% by volume or more, 70%by volume or more, 80% by volume or more, 90% by volume or more, orfurther 95% by volume or more.

Whether the base's structure is an austenite phase or not is prescribedby means of the above-described fourth condition substantially. That is,it is possible to narrow down a metallic structure to be obtained to anaustenite single phase by setting the intercept of a border line, whichdemarcates the lower limit of Ni_(eq) in the aforementioned fourthcondition, at 21.6. Note that it should be notified that the indexing ofB_(x), which designates the intercept of the border line that isindicated in the present invention, is an expediential one.

In the present invention, the upper limit of Ni_(eq) relative to theentire base is not limited essentially as far as Ni is a small amountrelative to the entire cast iron as specified in the second condition,because it is one of the objectives to obtain a cast iron that has abase being an austenite phase in ordinary-temperature region whilereducing the content of Ni to be made use of.

However, the elements other than Ni also exhibit limitations in terms oftheir solute contents in Fe. Moreover, those elements that get greateris not preferable, not only because costs have risen though thereduction of the Ni content can be intended, but also because desirablecast-iron structures have become less likely to obtain. Hence, in thepresent invention, although the upper limit of Ni_(eq) is set at 30%, itis preferable that the upper and lower limits of Ni_(eq) can be eitherone of 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, or further 20%.

Since issues that are similar to above apply to Cr_(eq) as well, theupper limit of Cr_(eq) is set at 13.5% in the present invention whileconsidering the generation circumstances of carbides that are believedto be the cause of fatigue-strength decline. However, it is preferablethat the upper and lower limits of Cr_(eq) can be either one of 12%,11%, 10%, 9%, 8%, 7%, 6%, 5%, or 4%.

In particular, such a case as where Cr_(eq) is from 5 to 8% and Ni_(eq)is 18% or more, or where Cr_(eq) is from 7 to 9% and Ni_(eq) is 13% ormore, is preferable, because the precipitation of lamellar carbides(including acicular carbides) does not occur or it is suppressed.

The “trace-amount modifier element” being referred to in the presentinvention is a trace-amount element that is effective in improvingcharacteristic. For example, it can be an element that contributes tometallic structure, such as spheroidizing graphite that crystallizes orprecipitates or increasing the number of the particles, and makingaustenite phase finer or stabilizing it. Moreover, it is permissiblethat it can also be an element that contributes to mechanicalcharacteristic, such as strength in room-temperature region orhigh-temperature region, high-temperature durability (i.e., creepstrength, and the like), toughness, and elongation. In addition, it ispermissible that it can even be an element that contributes to oxidationresistance, thermal expandability, thermal conductivity, workability,and so forth. Furthermore, it is permissible that it can also be anelement that contributes to castability, such as flowability at the timeof casting, and suppressing cast defects like cracks, shrinkage orpores.

As the “inevitable impurities,” the following are given: impuritiesbeing included in raw materials, impurities getting mingled or the likeat the time of casting, and so on. They are elements that are difficultto remove because of being costly, or due to technical reasons, etc. Forexample, as for such inevitable impurities, phosphorous (P), and thelike, are given.

In the present invention, the compositions of the trace-amount modifierelement and inevitable impurities are not limited in particular, becausethe compositions of the basic elements are important. For example, evenwhen being an austenitic cast iron in which no trace-amount modifierelement is included, not to mention the inevitable impurities, it fallswithin the range of the present invention. Note that, even when being anelement that can make a trace-amount modifier element, it is permissibleto treat it also as an inevitable impurity depending on its containedamount, or an application of the resulting cast iron, and the like.

When designating as “from ‘x’ to ‘y’” in the present specification, itincludes the lower limit, “x,” and the upper limit, “y,” unlessotherwise notified. Moreover, the elemental symbols or indexes (e.g.,Ni_(eq), Cr_(eq), C_(eq), C_(s), and the like), which are used in theconditional formulas or mathematical formulas in the presentspecification, index the contained amounts of those elements (% by mass)unless otherwise notified. In addition, the mark, “,” which is set forthin those conditional formulas or mathematical formulas, meansmultiplication (or product).

Further, for the component compositions that are used in the presentinvention, the following are given: a compositional range relative tothe entirety of a cast iron; and another compositional range relative tothe entirety of a base, namely, a part that constitutes that cast iron.However, the compositional range relative to the entire base is aportion that is relevant to the Ni_(eq) and Cr_(eq) which affect thebase's structure fundamentally. Therefore, compositions, which thepresent specification prescribes herein regarding portions other thanthe portion that is relevant to the Ni_(eq) and Cr_(eq), meancomponential compositions relative to the entirety of cast irons unlessotherwise notified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an XRD diagram on various cast irons with differentcompositions.

FIG. 2 is a correlation diagram for showing Cr_(eq)—Ni_(eq) regardingvarious cast irons with different compositions.

FIG. 3 is an XRD diagram on a cast iron (e.g., Test Specimen No. 2-2)that only had a distinct plate thickness to each other.

FIG. 4A is photomicrographs for showing metallic structures atrespective positions in a surface and the inside of a cast-productsample (e.g., Test Sample No. 3-1).

FIG. 4B is photomicrographs for showing metallic structures atrespective positions in a surface and the inside of a cast-productsample (e.g., Test Sample No. 3-2).

FIG. 5 is photomicrographs for showing metallic structures regardingrespective cast irons, namely, a basic material (FCDA-NiMn137 as perJIS) and a Cu-added material made by adding Cu to that base material,together with the Schaeffler's structural diagram on which theirpositions are designated.

FIG. 6 is photomicrographs for showing metallic structures ofcast-product samples (e.g., Test Sample Nos. 6-1 through 6-12).

FIG. 7 is a graph for illustrating a relationship between Cu additionamount and elongation in Fourth Test.

FIG. 8 is graph for illustrating a relationship between Cr additionamount and proof stress in Fourth Test.

FIG. 9 is an XRD diagram on cast irons with different compositions.

FIG. 10 is diagrams on correlations between the temperatures of variouscast irons and the linear expansion coefficients, wherein thecorrelation diagram labeled (a) in the same drawing corresponds to TestSpecimen No. 6-5; the correlation diagram labeled (b) in the samedrawing corresponds to Test Specimen No. 4-3; the correlation diagramlabeled (c) in the same drawing corresponds to Test Specimen No. R3; thecorrelation diagram labeled (d) in the same drawing corresponds to TestSpecimen No. R4; and the correlation diagram labeled (e) in the samedrawing corresponds to Test Specimen No. R6; respectively.

FIG. 11 is a bar graph for illustrating oxidized weight decrements ofvarious test specimens.

FIG. 12 is diagrams for illustrating correlations between oxidizedweight decrements and amounts of contained elements that are relevant tovarious test specimens, wherein labeled (a) in the same drawing isrelevant to the contained amounts of Cr; and labeled (b) in the samedrawing is relevant to the contained amounts of Ni.

FIG. 13 is diagrams for illustrating correlations between oxidizedweight decrements and amounts of contained elements that are relevant tovarious test specimens, wherein labeled (a) in the same drawing isrelevant to the contained amounts of Mn; and labeled (b) in the samedrawing is relevant to the contained amounts of Cu.

FIG. 14 is a bar graph for illustrating Charpy-impact values of varioustest specimens.

FIG. 15 is a diagram for illustrating a correlation betweenCharpy-impact values and contained Cr amounts that are relevant tovarious test specimens.

FIG. 16 a bar graph and dispersion diagram for illustrating 0. 2% proofstresses and fracture elongations of various test specimens at 800° C.

FIG. 17 is diagrams for illustrating correlations between fractureelongations and amounts of contained elements that are relevant tovarious test specimens, wherein labeled (a) in the same drawing isrelevant to the contained amounts of Cr; and labeled (b) in the samedrawing is relevant to the contained amounts of Cu.

FIG. 18 is a bar graph for illustrating hardnesses of various testspecimens.

FIG. 19 is a photograph for showing misrun defects which make an indexfor evaluating the various test specimens' molten-metal runningproperties.

FIG. 20 is a bar graph for relatively evaluating molten-metal runningproperties exhibited by various test specimens, and illustrates themwith respect to that of Test Specimen No. 7-1 being taken as “1.”

FIG. 21 is a bar graph for illustrating thermal-fatigue lives of varioustest specimens.

FIG. 22 is a bar graph for illustrating thermal-fatigue lives of varioustest specimens.

FIG. 23 is a graph for illustrating correlations between values ofhardness rise and plate thicknesses of a test specimen when variouselements were added in an amount of 1%.

FIG. 24 is photographs for explaining a method of quantifying shrinkagemagnitudes in various test specimens.

FIG. 25 is a bar graph for relatively evaluating shrinkage magnitudesexhibited by various test specimens, and illustrates them with respectto that of Test Specimen No. R3 being taken as “1.”

FIG. 26 is a graph for illustrating correlations between average linearexpansion coefficients of various test specimens and widths of heatingtemperatures.

FIG. 27 is a bar graph for illustrating average linear expansioncoefficients of various test specimens.

FIG. 28 is a bar graph for illustrating thermal conductivities ofvarious test specimens.

FIG. 29 is a bar graph for illustrating oxidized weight decrements ofvarious test specimens at respective heating temperatures.

FIG. 30 is a bar graph for illustrating proof stresses of various testspecimens at respective temperatures.

FIG. 31 is a bar graph for illustrating tensile strengths of varioustest specimens at respective temperatures.

FIG. 32 is a bar graph for illustrating fracture elongations of varioustest specimens at respective temperatures.

FIG. 33 is a bar graph for illustrating thermal-fatigue lives of varioustest specimens under respective conditions.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be explained in more detail while givingembodiment modes. Note that, although austenitic cast irons will bedealt with mainly in the present specification to explain the presentinvention, it should be notified that their contents can be appliedappropriately not only to the austenitic cast product (including thecomponent part for exhaust system) according to the present inventionbut also to the process for manufacturing the same. Moreover, whetherany one of the embodiment modes is considered best or not depends onsubject matters, their required performance, and the like.

(Composition)

(1) Basic Elements

An austenitic cast iron according to the present invention comprisesbasic elements, and Fe, namely, the balance. The basic elements comprisesix types of elements, namely, C, Si, Cr, Ni, Mn and Cu. However, in acase where the austenitic cast iron does not include any Cusubstantially, five elements, namely, C, Si, Cr, Ni and Mn, make thebasic elements. Hereinafter, the actions or functions of each of theserespective elements, and their suitable compositions will be explained.

(i) C and Si

(a) C drops the molten temperature of Fe, and enhances the flowabilityof molten metal (including modifier-free molten metal). Consequently, itis an indispensable element for ferrous casting. Since C in Fe—C systemalloys exceeds the maximum solid-solubility limit so that cast irons areaccompanied by eutectic solidification, the lower limit of C can be1.7%, 1.8%, 1.9%, 2%, or 2.1% fundamentally; and its upper limit can be5%, or further 4.3%. Note that C that exceeds the solid-solubility limitcrystallizes as graphite.

When C is too little, no preferable castability can be obtained becausethe flowability of molten metal has declined. When C is too much, theresulting base's structure decreases, and thereby the resultingaustenitic cast iron's mechanical characteristics, and the like,decline. Moreover, cast defects, such as shrinkage cavities, becomelikely to occur at the time of casting. Hence, it is more preferablethat the lower limit of C can be 2%, or 2.5%, and that its upper limitcan be 5%, or 3.5%.

(b) Note that the solute carbon content (C_(s)), which becomes necessaryfor calculating the Ni_(eq) being referred to in the present inventioncan be found essentially by analyzing the composition of Fe basestructure, or by subtracting a total amount of C, which crystallized orprecipitated graphite and carbides, such as cementite (Fe₃C), haveconsumed, from the entire amount of blended C.

However, since this C_(s) is a trace amount, it is difficult to analyzeit accurately. Moreover, it has been understood empirically that C_(s)falls in a virtually constant range. Hence, even when assuming thatC_(s)=0.03% in calculating Ni_(eq) to finding it, it is the actualityhowever that errors that occur in the resulting Ni_(eq) are small tosuch an extent that they can be negligible substantially. Hence, in thepresent invention, Ni_(eq) has come to be found on the assumption thatC_(s)=0.03%.

Note that 0.03% is the solid-solubility limit of C to a (ferrite) phasein the Fe—C binary system phase diagram. Since it is presumed fromviewing the phase diagram that the solute content to γ (austenite) phasecan be this solute content or more, the value of C_(s) has come to beassumed to be 0.03% as the minimum value of the solute content.

(c) Si lowers the eutectic temperature of metastable system, facilitatesthe eutectic crystallization of γ Fe-graphite, and then contributes tothe crystallization of graphite. Moreover, Si forms passive films, whichcomprise silicon oxide in the vicinity of crystallizing graphite'ssurface, and thereby enhances the oxidation resistance of cast iron.

When Si is too little, no such effects can be obtained sufficiently; andSi being too much is not preferable because it causes the decline ofelongation and the worsening of machinability. Hence, it is preferablethat the lower limit of Si can be 2%, 3%, or further 3.5%. It ispreferable that the upper limit of Si can be 6%, 5.5%, 5%, or further4.5%.

(d) By the way, Si has an action of shifting a eutectic carbon contentof Fe—C system toward lower-concentration side, and then a carbonequivalent (C_(eq)=C+Si/3) in which an Si content is taken account intoa C content is used as an index. Hence, it is more preferable that thelower limit of C_(eq) can be set at 2.1%, at 2.5%, or further at 3%. Itis more preferable that its upper limit can be set at 5%, or at 4.3%,namely, the eutectic point in the Fe—C system phase diagram, or furtherat 3.5%.

(ii) Cr

Cr binds with carbon in cast-iron base to precipitate carbides therein,and then improves the high-temperature proof stress of cast iron bymeans of the precipitation strengthening of the resulting base.Moreover, it makes it possible to improve the oxidation resistancebecause it forms passive films, which comprise dense and fine chromiumoxides in the vicinity of the resulting cast iron's surface.

Moreover, Cr being too much is not preferable because carbides increaseso that the toughness and workability decline. Hence, it is preferablethat the lower limit of Cr can be 0.1%, 0.3%, 0.5%, 0.7%, 1%, 1.2%, orfurther 1.5%. It is preferable that the upper limit of Cr can be 6%, 5%,4%, 3%, 2.5%, or further 2%.

Incidentally, when the present inventors analyzed cast irons in which Crwas contained in an amount of from 9 to 15% relative to the entire castirons, it was understood that many Cr—Mn system carbides crystallized orprecipitated so that the resulting cast irons are chilled (or carbidize)as a whole. Therefore, when the Cr content exceeds 9%, the austeniticcast iron being referred to in the present invention is less likely tobe obtainable.

(iii) Ni

Ni is an effective element in the austenitization of base's structure.When Ni is too little, it is hard to obtain stable austenite phase. Onthe other hand, when Ni becomes too much, making austenitic cast ironinexpensive by means of the reduction of Ni content, namely, one of theobjectives of the present invention, cannot be intended.

Hence, it is preferable that the lower limit of Ni can be 12%, 11%, 10%,9%, 8%, or further 7%. Moreover, it is preferable that the upper limitof Ni can be 15%, 14%, 13%, 12%, 11%, 10%, or further 9%.

(iv) Cu and Mn

(a) Cu and Mn are effective elements in the austenitization of base'sstructure, as well as Ni.

Note herein that the equation for calculating Ni_(eq) according to thepresent invention can be turned into 0.5Mn+Cu=Ni_(eq)−Ni−30C₅.

And, the upper limit of Ni according to the present invention is nohigher than 15%. Moreover, regardless of the total contained amount ofC, C_(s) falls within a virtually constant range (e.g., from 0 to 0.8%).The C_(s) content falls in such a range, because the solute amount of Cin γ Fe declines from 2.1%, namely, the maximum, to 0.8% approximatelyas being accompanied by temperature decline in the Fe—C binary systemphase diagram.

Incidentally, although “0.5,” namely, the coefficient of Mn, is onewhich is specified in the Schaeffler's structural diagram, “1,” namely,the coefficient of Cu, is one which the present inventors had come toknow totally newly as a result of their earnest studies through avariety of experiments, and the like. The background on this issue willbe described in detail as follows.

Cast-iron test specimens were made ready, cast-iron test specimens whichcomprised the following, respectively: a basic material (Fe-3% C-2.3%Si-13% Ni-7% Mn equivalent to FCDA-NiMn137 as per JIS, that is,equivalent to later-described Test Specimen No. R2 in Table 1A); and aCu-added material in which Cu was added in an amount of 6.5% to thisbasic material (equivalent to later-described Test Specimen No. 1-1 inTable 1A). The following are shown in FIG. 5 all together: structuralphotographs in which these were observed; and their positions, whichwere findable from their respective compositions, on the Schaeffler'sstructural diagram.

In the case of the basic material, Ni_(eq)=18.2, and Cr_(eq)=4.1 can bederived from its own composition. When plotting these on theSchaeffler's structural diagram, it is expected that the basic materialhas a quasi-austenite structure of “A”+“M.” This fact was alsoascertained from the structural photograph of the basic material. Thatis, it was ascertained that the basic material's base comprised anaustenite phase (or γ phase), and lamellar carbides that were formed of2 phases, namely, carbide layers, which were seemed to precipitate fromthat γ phase during the process of cooling at the time of casting, andan α phase.

Note that, compared with such a martensite structure as can be observedin usual ferritic cast iron, like that of FCD4500 as per JIS, thethicknesses of the carbide layers in the basic material became greaterand the intervals between the layers became wider, it is believedbecause of the fact that the basic material contained Mn that is morelikely to generate carbides (that is, whose free energy is lower) thanis Fe comparatively.

By the way, when analyzing the composition of the Cu-added material inwhich Cu was added to the basic material, the compositions of the majorelements were as follows: 2.3% Si; 10.4% Ni; 6.5% Mn, and 7.2% Cu. Whenapplying these compositions to the conventional Schaeffler's structuraldiagram, they make Ni_(eq) (=Ni+30·C_(s)+0.5·Mn)=14.7, and Cr_(eq)(=Cr+1.5Si)=3.5. When plotting these on the Schaeffler's structuraldiagram as they were, the resulting position falls in the martensiteregion (or “M” region).

However, no lamellar carbides like those in the basic material were seenin the actual structural photographs of the Cu-added material. That is,it is believed that the base turned into an austenite single phasevirtually by means of adding Cu. This is speculated because of thefollowing: the lamellar carbides disappear by means of the Cu additionso that γ phase has stabilized.

According to this result, the base of the Cu-added material should cometo be positioned essentially in the austenite single phase region (i.e.,“A” region), so to speak, on the Schaeffler's structural diagram. WhenCr_(eq)=3.5, Ni_(eq) being 22.5 or more enters the “A” region on theconventional Schaeffler's structural diagram.

If so, in the case of the aforementioned Cu-added material, adiscrepancy, namely, at least ΔNi_(eq)=22.5−14.7=7.8, comes to arisebetween Ni_(eq), which is found calculationally from the analyzedcompositions, and Ni_(eq), which is assumed from observing the actualstructure. It is apparent that the cause of arising such a discrepancyresults from the addition of Cu from the above-described background.Therefore, it is believed that the added Cu has facilitated theaustenitization of the base of the Cu-added material and has thenstabilized an austenite phase. To put it in other words, it is possibleto say that Cu has acted in the direction of augmenting Ni_(eq). And, aninfluential proportion to Ni_(eq) by means of Cu becomes(ΔNi_(eq)/Analyzed Contained Cu Amount)=7.8/7.2=1.08, and is about “1”approximately at a moderate estimate. And, think of the background,namely, the metallic structure changes from “A”+“M” to “A” by means ofthe addition of Cu, it is hardly think of the influential proportion toNi_(eq) by means of Cu that becomes far greater than “1.” Hence, in thepresent invention, the coefficient of Cu is set at “1” in calculatingNi_(eq).

(b) As described above, in addition to solving into base and thenstabilizing austenite structure as well as Ni, Cu refines thecrystalline grains in base's structure to improve the high-temperatureproof stress. Moreover, it is an effective element in improving theoxidation resistance and corrosion resistance as well.

However, when Cu becomes excessive, the peritectic structure of Cuappears so that the spheroidizing of graphite is hampered to decline thestrength and the like of cast iron. Moreover, Cu that becomes excessiveis not preferable, because the peritectic structure of Cu appears andthereby the elongation performance at the time of high temperaturesworsens. Hence, it is preferable that the lower limit of Cu can be 0%,0.1%, 0.3%, 0.5%, 0.7%, 1%, or further 1.2%. It is preferable that theupper limit of Cu can be 3%, 2.5%, 2%, 1.8%, or further 1.8%. Note that,as described above, in a case where an austenitic cast iron according tothe present invention comprises Cu as an essential element, the lowerlimit of Cu being 0% means that 0%<Cu. On the other hand, in anothercase where Cu is not an essential element, the lower limit of Cu being0% means that 0%≦Cu.

(c) In addition to being effective in the stabilization of austenitestructure, Mn is also an effective element in the removal and the likeof S that becomes the cause of flowability worsening and embrittlement.

When Mn is too little, these effects cannot be obtained sufficiently.When Mn becomes excessive, Mn carbides increase to cause the decline ofthe toughness and so forth of cast iron, or the decline of heatresistance. Moreover, that is not preferable, because gas defects, suchas blow holes, become likely to occur. Hence, it is preferable that thelower limit of Mn can be 0%, 0.1%, 0.5%, 1%, 2%, 2.5%, 3%, 4%, orfurther 5%. It is preferable that the upper limit of Mn can be 9%, 8%,7%, or further 6%.

(2) Trace-amount Modifier Element

(a) It is preferable to make a trace-amount element be contained inorder to improve a variety of characteristics, such as the metallicstructure of austenitic cast iron (or cast product), the oxidationresistance, the corrosion resistance, the strength inordinary-temperature region or high-temperature region, mechanicalcharacteristics like strength or toughness, and electriccharacteristics. Austenitic cast irons that include such a modifierelement also falls within the limitations of the present inventionnaturally as far as the basic elements fall within the above-describedranges.

The trace-amount modifier element can be the following, forinstance:magnesium (Mg), rare-earth elements (R.E.), aluminum (Al),calcium (Ca), barium (Ba), bismuth (Bi), antimony (Sb), tin (Sn),titanium (Ti), zirconium (Zr), molybdenum (Mo), vanadium (V), tungsten(W), niobium (Nb), or nitrogen (N), and the like.

The content of each of these elements can be adjusted appropriatelydepending on characteristics that are required for austenitic castirons. However, from the viewpoints of influences and so forth to costsand the compositions of the basic elements, it is preferable that thetrace-amount modifier elements can be 1% or less, 0.8%, or further 0.6%or less in a total contained amount.

An added trace-amount modifier element might possibly disappear and thelike during casting, because the melting point is lower than that of Fe.Accordingly, the content of each of the respective elements does notnecessarily coincide with the total addition amount of that element.Consequently, as far as being effective in the improvement and so forthof cast structure, it is permissible that the contained amount of thattrace-amount modifier element can be at the minimum level that isdetectable.

(b) A representative trace-amount modifier element is each of therespective elements that are included in an inoculant agent, whichfacilitates the crystallization of graphite within Fe base, or aspheroidizing agent, which facilitates the spheroidizing of resultantcrystallized graphite. An auxiliary agent, such as an inoculant agent orspheroidizing agent, is blended at the time of preparing a molten metal,or is added appropriately at the time of casting. However, its containedelements and the contained amounts of the respective elements are notfixed, but vary greatly. That is, it is the actual situation howeverthat they are sought by trial and error in order to obtain desired caststructures (e.g., the configurations of crystallizing graphite or thenumber of their particles especially), and the like. Therefore, it isdifficult to clearly identify the type of the trace-amount modifierelements and their contained amounts. And, adhering to the type of thetrace-amount modifier elements and the contained amounts is against thetrue aim of the present invention.

However, Mg and R.E. (e.g., cerium (Ce) especially) have been knownpublicly as spheroidizing agents for crystallizing graphite. Hence, inthe case of the austenitic cast iron according to the present inventionas well, it is preferable to include Mg in an amount of from 0.01 to0.1% and/or Ce in an amount of from 0.005 to 0.05%, Mg and Ce whichserve as a trace-amount modifier elements respectively, relative to theentire cast iron being taken as 100%.

Here, since Mg is likely to disappear from inside high-temperature motelmetals, it is preferable that the addition amount can be adjusted tosuch an extent that its lower limit becomes 0.02%, or further 0.03%,relative to the entire cast iron being taken as 100%. Although the upperlimit of the contained Mg amount is not limited in particular as far asit does not affect the compositions of the basic elements, it can be, inactuality however, 0.07%, or further 0.06%, relative to the entire castiron being taken as 100%.

Since Ce, namely, an R.E., is expensive, and moreover since the effectof spheroidizing is obtainable even when being included in a smallamount, it is preferable that the upper limit of Ce can be 0.03%, orfurther 0.01%, relative to the entire cast iron being taken as 100%.Although the lower limit of Ce is not limited in particular as far as itfalls in a range in which the effect of serving as a spheroidizing agentis obtainable, the lower limit thereof can be, in actuality however,0.007%, or further 0.008%, relative to the entire cast iron being takenas 100%.

(3) Inevitable Impurities

As inevitable impurities, phosphorous (P), and sulfur (S) are given, forinstance. P is harmful to the spheroidizing of graphite, and moreoverprecipitates in crystal grain boundaries to decline oxidation resistanceand room-temperature elongation. S is also harmful to the graphiticspheroidizing. Therefore, it is preferable that each of these inevitableimpurities can be set at 0.02% or less, or further 0.01% or less.

(Production Process)

(1) Since the present invention is a manufacturing process foraustenitic cast iron, it is equipped with a molten-metal preparationstep, a pouring step, and a solidification step that are like those asdescribe above. However, in the case of manufacturing members, such asautomotive component parts for which high reliability is required, withcast products, it is required that the austenitic cast iron according tothe present invention be a spheroidal graphite cast iron. Hence, it isdesired to crystallize a large number of spheroidal graphite finely andminutely within base that comprises austenite phase, and accordingly theblend or addition of auxiliary agent, such as an inoculant agent orspheroidizing agent, is done.

For instance, these auxiliary agents have been blended beforehand fromthe stage of the molten-metal preparation step. However, in order toprevent the disappearance of those auxiliary agents, and such a fadingphenomenon that the effects of the auxiliary agents reduce as beingaccompanied by the elapse of time, and in order to make the auxiliaryagents function effectively, it is more suitable to first prepare such amolten metal, which comprises the basic elements, previously (i.e., amodifier-free-molten-metal preparation step), and then to be equippedwith an auxiliary-agent addition step of blending an auxiliary agentwith or adding it to that modifier-free molten metal directly orindirectly.

Here, the case of adding an auxiliary agent “directly” is such a casewhere it is added to the modifier-free molten metal before pouring itinto a casting die, and the like. Moreover, the case of adding or thelike an auxiliary agent “indirectly” is such a case where it is chargedin a cavity of casting die in advance, and so forth. For example, whenbeing the case of inoculating, it is permissible to do it by any one ofthe following: ladle inoculation, inoculating inside casting die, wireinoculation, and so on. It is the same in the case of spheroidizingtreatment, too.

After all, since ordinary cast products are cast by injecting the moltenmetal (or modifier-free molten metal) into a ladle from a meltingfurnace or retaining furnace and then pouring that molten metal into acasting die, it is even permissible that the addition of an auxiliaryagent can be carried out at any one of those stages. Moreover, it ispermissible that the auxiliary agent can have any one of powdery shapes,granular shapes, wired shapes, and the like. Note that, although theauxiliary agent can be represented by inoculant agents and spheroidizingagents, it can be additive agents other than these.

(2) In view of the composition elementally, it is preferable that theinoculant agent can comprise one or more members of Si, Ca, Bi, Ba, Al,Sn, Cu, or R.E., for instance. To be concrete, the following inoculantagents are available: Si—Ca—Bi—Ba—Al-system ones,Si—Ca—Bi—Al-R.E.-system ones, Si—Ca—Al—Ba-system ones, Si—Sn—Cu-systemones, and the like. The addition amount or blended amount of inoculantagent is determined in consideration of the disappearance, the fadingphenomenon, and so forth. Hence, it is preferable to set so that thetotal addition amount becomes from 0.05 to 1%, for instance, when theentire modifier-free molten metal is taken as 100%.

In view of the composition elementally, it is preferable that thegraphite spheroidizing agent can comprise one or more members of Mg, andR.E., for instance. To be concrete, the following spheroidizing agentsare available: Mg-R.E.-system ones, Mg simple substance, R.E. simplesubstances such as mish metal (or Mm), Ni—Mg-system ones,Fe—Si—Mg-system ones, and the like. The addition amount or blendedamount of spheroidizing agent is also determined in consideration of thedisappearance, the fading phenomenon, and so forth. For example, it ispreferable to add a spheroidizing agent so that a residual Mg content(that is, a content of Mg that remains in a prepared cast iron) becomesfrom 0.01 to 0.1%, more preferably, from 0.03 to 0.08%, when the entiremodifier-free molten metal is taken as 100%.

Note that, as far as the configuration or number of particles ofcrystallizing graphite falls within the desirable range, it is optionalthat to what extent any one of the inoculant agents or spheroidizingagents is added.

(Austenitic Cast Product)

(1) Although the austenitic cast product according to the presentinvention is members with desirable configuration that comprise theabove-described austenitic cast iron according to the present invention,it is needless to say that their configurations, wall thicknesses, andthe like, do not matter at all.

Here, although it is also possible to think of that the thickness,configuration, size, casting designs and the like of cast product haveinfluences on the structure, cast defects and so forth of austeniticcast iron, it had been ascertained that, in the case of the austeniticcast product according to the present invention, the base is a stableaustenite phase. Moreover, even in a case where the thickness of castproduct is so thin that the molten metal is quenched and then rapidlysolidifiedpartially, the present inventors had ascertained already thatit is possible to obtain desired spheroidal graphite cast irons byadjusting the addition method of an auxiliary agent or the additiontiming appropriately.

(2) The structure of austenitic cast iron is divided roughly into a basestructure, and a eutectic structure. A base structure according to thepresent invention comprises an austenite phase of Fe. A eutecticstructure according to the present invention is graphite.

Generally speaking, although cast irons are classified variouslydepending on the forms of crystallizing graphite, being spheroidalgraphite cast irons is preferable because they are good in terms ofevery one of characteristics, such as mechanical characteristics,compared with those of the other cast irons. Hence, it is suitable thatthe austenitic cast iron according to the present invention can alsocomprise a spheroidal graphite cast iron.

The structure of spheroidal graphite cast iron is indexed by means of aspheroidized proportion of graphite and the number of graphite particlesin general. First of all, actual austenitic cast products that are goodin terms of characteristics exhibit such a spheroidized proportion ofgraphite, which crystallized or precipitated in the base, as 70% ormore, 75% or more, 80% or more, or further 85% or more. Next, thegreater the number of graphite particles that have crystallized orprecipitate is, themore desirable it is. For example, in a section whosecast-product wall thickness is 5 mm or less, it is suitable that thenumber of graphite particles whose particle diameter is 10 μm or morecan be 50 pieces/mm² or more, 75 pieces/mm² or more, or further 100pieces/mm² or more. Note that it is preferable that spheroidal graphitecan be dispersed within base very finely. Moreover, in a section whosecast-product thickness is 5 mm or less, it is suitable that the numberof graphite particles whose particle diameter is 5 μm or more can be 150pieces/mm² or more, 200 pieces/mm² or more, 250 pieces/mm² or more, orfurther 300 pieces/mm² or more. Note that it is preferable thatspheroidal graphite can be dispersed within base very finely.

Note that the spheroidized proportion of graphite can be measured bymeans of “G5502 10.7.4” as per JIS or thespheroidized-graphite-proportion judgment testing method as per old JIS“5502” (or the NIK method). Moreover, the number of graphite particlescan be measured by means of counting the number of graphite particlesper unit area.

(3) Not only the austenitic cast iron according to the present inventionis excellent in terms of strength, toughness, workability and the likein ordinary-temperature region, but also it is excellent in terms ofheat resistance such as highly resistant to oxidation andhigh-temperature proof stress. Hence, the austenitic cast productaccording to the present invention that comprises this cast iron issuitable for exhaust-system component parts for automobile, and soforth. To be more concrete, the housings of turbocharger, exhaustmanifolds, catalyst cases, and so on. This is because not only thesecomponent parts are always exposed in high-temperature environments thatresult from high-temperature exhaust gases, but also they are exposed tothe sulfur oxides, nitrogen oxides etc. in the exhaust gases.

The austenitic cast product according to the present invention is notlimited to members that are made use of in such high-temperature region.It is natural that it is utilizable in such members as well that aremade use of in ordinary-temperature region or warm region. Inparticular, since the austenitic cast product according to the presentinvention can be manufactured at lower cost than conventional ones, therange of its utilization can also be expanded. Moreover, the field ofutilization is not limited to the field of automobiles and the field ofengines, and the austenitic cast product according to the presentinvention can be utilized for various kinds of members, too.

Examples

The present invention will be explained more concretely while givingexamples.

(First Test)

(1) Manufacturing Method of Test Specimens

Raw materials, which included at least one or more members of C, Si, Cr,Ni, Mn and Cu (i.e., basic elements) and the balance of Fe, were blendedand mixed variously, and they were air melted with a high-frequencyfurnace, thereby obtaining 47-kg molten metals (i.e. , a molten-metalpreparation step). Each of these molten metals was poured into a castingdie (e.g., sand die) that had been made ready in advance (i.e., apouring step). On this occasion, they were tapped at about 1,550° C.,and were poured at about 1,450° C. Moreover, the after-pouring moltenmetals were solidified by natural cooling (that is, in a state of ascast), thereby obtaining test specimens with said configuration (or castproducts) (i.e., a solidification step).

Note that the addition of an auxiliary agent, such as an inoculant agentand spheroidizing agents, was also carried out when casting therespective test specimens. The addition of the inoculant agent wascarried out by adding “CALBALLOY” (containing Si—Ca—Al—Ba) produced byOSAKA SPECIAL ALLOY Co., Ltd. in an amount of 0.2% by mass with respectto the modifier-free molten metals being taken as 100%. The addition ofthe spheroidizing agents was carried out by adding the following to themodifier-free molten metals: an Mg simple substance in an amount of 4%by mass; R.E. (e.g., misch metal) in an amount of 1.8%; and an Sb simplesubstance in an amount of 0.005% by mass; with respect to themodifier-free molten metals being taken as 100%. Note that the amount ofMg was great because the disappearance and the like were considered.

The casting die being used herein was a sand die whose size was 50 mm inwidth×180 mm in overall length, and from which a stepped plate-shapedcast product was obtainable, stepped plate-shaped cast product whoseheight (or thickness) changed in five stages in the following order: (i)50 mm (50 mm in length)→(ii) 25 mm (45 mm in length)→(iii) 12 mm (40 mmin length)→(iv) 5 mm (25 mm in length)→3 mm (20 mm in length).

Moreover, for the measurements of proof stress and tensile strength,type-“B” “Y”-shaped blocks as per JIS were prepared by means of moldcasting, and then φ6 round rod test specimens were prepared from therectangular vertical cross-sectional part of the resulting “Y”-shapedblocks.

(2) Measurement of Test Specimens

Five types of test samples (e.g., Nos. 1-1 through 1-5) having differentblended compositions were manufactured by means of the aforementionedmanufacturing process. Samples, which were collected from a section ofthe respective test specimens with 5-mm thickness, were subjected to thefollowing analyses.

(i) The respective samples were analyzed compositionally by mean of anX-ray micro analyzer (or EPMA), thereby obtaining the analyzedcompositions of the entire cast irons and the analyzed compositions ofthe Fe bases. The thus obtained compositions of the basic elements areshown in Table 1A.

Note that the designation, “−” in Table 1A, specifies either one ofbeing unblended, being unanalyzed or unmeasured, or being unable toanalyze or unable to measure. This applies similarly to other tables inthe present specification, namely, to Tables 1B through 4B.

(ii) Moreover, FIG. 1 illustrates an analyzed diagram (or XRD) in whichthe respective samples were analyzed by X-ray diffraction. Forreference, XRDs on representative cast irons, which have been said to beaustenitic cast irons (e.g., Reference Examples: R1 and R2), are alsoillustrated on FIG. 1 all together. Further, austenite proportions,which were found based on those XRDs, are also shown in Table 1 alltogether.

(iii) Furthermore, the Ni_(eq) and Cr_(eq) that are referred to in thepresent invention were calculated from the Fe-base composition of eachof the samples, and were then shown in Table 1A. Each of those Ni_(eq)and Cr_(eq) were plotted on the correlation diagram that is illustratedin FIG. 2. Test Specimen Nos. 1-1 through 1-5 are designated with marks. The representative conventional cast irons (e.g., R3: D-5S, andR4: D-2) were designated with ◯ marks. Here, the Ni_(eq) was found whileassuming that C_(s)=0.03%, because it is difficult to analyze the C_(s)directly.

Note that, in order to discuss whether the obtained cast irons are anaustenitic cast iron or not based on the Ni_(eq−Cr) _(eq) correlationdiagram illustrated in FIG. 2, that is, in order to discuss how muchpercentages an austenite proportion account for in the Fe bases,strictly speaking, it is necessary to analyze the compositions of the Febases from which carbides and graphite are removed. Hence, regardingTest Specimen Nos. 1-1 through 1-5, the Ni_(eq) and Cr_(eq) that wentwith this way of thinking were calculated, and are then shown in Table1A.

However, even when being those which are said to be an austenitic castiron in general, very few of them comprise a 100%-austenite single-phasebase as described above. And, excepting C most of which crystallizes orprecipitates as graphite, there is a correlation between the analyzedcompositions of the Fe bases and the analyzed compositions of the entirecast irons as far as they fall within the compositional ranges that areprescribed in the present invention, and there is no such a greatdiscrepancy between both of them.

Hence, regarding Reference Example Nos. R1 through R6 in Table 1A andtest specimens other than Nos. 1-1 through 1-5 therein, the Ni_(eq) andCr_(eq), which were calculated using the analyzed compositions of theentire cast irons that served as the substitutes for the analyzedcompositions of the Fe bases, are given, for reference for the sake ofconvenience.

(iv) In the measurements of proof stress and tensile strength, testswere carried out at 150° C. and 800° C. in conformity to “G0567” as perJIS. The resultant measured data on the proof stress and tensile stressare shown in Table 1A and Table 1B all together.

(3) Evaluation

(i) From Table 1A and FIG. 1, it is understood that, in any one of thecases of Test Specimen Nos. 1-1 through 1-5 in which the Ni contentswere reduced, austenite phases (or γ phases) appeared in the same mannerin R1 and R2, namely, conventional austenitic cast irons.

In particular, in the cases of Test Specimen Nos. 1-1 through 1-3, it isunderstood that the bases' structure turned into an austenite singlephase virtually when the contained Ni amounts were at around 10% at thehighest.

(ii) Moreover, it was understood from the analyzed compositions of theFe bases in Table 1A that Si solves into Fe in an amount of up to 5.1%at least, Cu solves thereinto in an amount of up to 7.2% at least, andMn solves thereinto in an amount of up to 14.5% at least. Moreover, withreference to the Fe—Ni binary system phase diagram, it is possible tosay that Ni falling within the range according to the present inventioncompletely solves into Fe.

(Second Test)

(1) Manufacturing Method of Test Specimens

Raw materials, which included at least one or more members of C, Si, Cr,Ni, Mn and Cu (i.e., basic elements) and the balance of Fe, were blendedand mixed variously, and they were air melted with a high-frequencyfurnace, thereby obtaining 47-kg stock molten metals (i.e., amodifier-free-molten-metal preparation step). Each of thesemodifier-free molten metals was poured into a casting die (e.g., sanddie) that had been made ready in advance (i.e., a pouring step). In thepresent test, an inoculant agent and spheroidizing agents that compriseda variety of compositions had been charged into the casting die inadvance (i.e., an auxiliary-agent addition step). The other steps werethe same as those in the case of First Test.

(2) Measurement of Test Specimens

Thirteen types of Test Specimen Nos. 2-1 through 2-13 having differentblended compositions were manufactured by means of the aforementionedmanufacturing process. Samples, which were collected from a section ofthe respective test specimens with 12-mm thickness, were subjected tothe following analyses.

(i) In the same manner as in the case of First Test, the analyzedcomposition and austenite proportion of each of the samples were found.These results are shown in Table 2A and Table 2B.

(ii) Regarding each of the samples, a structural observation was carriedout bymeans of the optical-microscope photograph, thereby examining thecrystallized form of eutectic graphite. The spheroidized graphiteproportion was found by means of the judgment testing method accordingto “G5502 (or the NIK method)” as per old JIS.

The number of graphite's particles was found by counting those whoseparticle diameters were 10 μm or more in a 4.8-mm² area.

Further, a hardness (Hv at 20 kgf) making an index of cast product'sstrength, and the like, was measured as well . These results are shownin Table 2B all together.

(iii) Furthermore, in the same manner as First Test, the Ni_(eq) andCr_(eq) were calculated from the analyzed composition of each of theentire samples, and were then shown in Table 2B. Each of these Ni_(eq)and Cr_(eq) was plotted with “+” marks on the structural diagram in FIG.2 in a superimposed manner. The C_(s) was treated in the same manner asin the case of First Test.

(iv) In the same manner as in the case of First Test, the heat-resistantstrength of each of the samples was found, and was then shown in Table2B all together.

(3) Evaluation

(i) As can be understood when examining Table 2B carefully, it isunderstood that it is possible to obtain bases having austenite phasesvirtually even when the Ni contents are less.

By the way, according to researches by the present inventors, it wasascertained that bases' structure does not affect the thickness of testspecimens. To put it differently, it is possible to say that the castiron according to the present invention is not affected by thesolidifying rate, and the like, and thereby stable austenite phases areformed. An XRD that evidences this issue is illustrated in FIG. 3. TheXRD in FIG. 3 was obtained by subjecting the 5-mm-thickness section and12-mm-thickness section of Test Specimen No. 2-2 to X-ray diffraction.

(ii) However, as can be understood when examining Table 2B carefully,those being austenitic cast irons do not necessarily turn intospheroidal graphite cast irons. And, there were such cases that thespheroidized proportions, and the number of spheroidal-graphiteparticles were low.

Therefore, in order not only to turn the cast iron's base structure intoan austenite phase but also to obtain a eutectic structure in whichspheroidal graphite is crystallized adequately, it becomes necessary notonly to set the compositions of the basic elements in a molten metal ormodifier-free molten metal within the ranges according to the presentinvention but also to take individual measures in compliance with castproducts' configuration, molten metals' composition, and the like. Forexample, it is desired to select the types of auxiliary agent, theaddition amounts, and so forth, appropriately in compliance with castproducts' configuration, molten metals' composition, and so on. Hence,examples in which the present inventors optimized the eutecticstructures individually will be specified in later-described Third Test.

(iii)) As can be understood from Table 2B, it is also appreciated thatany one of the test specimens being directed to the present inventionhad strength (or hardness) and heat-resistance strength that wereequivalent to or more than those of conventional austenitic cast irons(e.g., Reference Examples R3 and R4). In particular, the test specimensbeing directed to the present invention exhibited larger proof stressesat 800° C., which matter in view of practical perspective, than did theconventional austenitic cast irons. As a result of this, it was possibleto ascertain that the austenitic cast iron being directed to the presentinvention has high heat resistance that is equivalent to or more thanthose of conventional ones.

(Third Test)

(1) Manufacturing Method of Test Specimens Although the compositions ofthe basic elements, and the types and addition amounts of the auxiliaryagents were changed, the others were set in the same manner as those ofSecond Test and then two types of test specimens, namely, Test SpecimenNo. 3-1 and Test Specimen No. 3-2, were manufactured.

An inoculant agent being added in Test Specimen No. 3-1 was “TOYOBARONBIL,” namely, 74.18Si-1.23Ca-0.55Ba-0.72Bi-0.51Al—Fe, produced by TOYODENKA Co., Ltd. This one was added in a proportion of 0.2% by mass withrespect to the modifier-free molten metal.

Moreover, used spheroidizing agents were the following: an Mg simplesubstance in an amount of 4% by mass; R.E. (e.g., misch metal) in anamount of 1.8% by mass; and an Sb simple substance in an amount of0.005% by mass; and those were added in the respective proportions withrespect to the modifier-free molten metals. Note that the amount of Mgwas great because the disappearance and the like were considered.

An implant agent being used in Test Specimen No. 3-2 was said “TOYOBARONBIL.” This one was added in a proportion of 0.4% by mass with respect tothe modifier-free molten metal. As for spheroidizing agents, thefollowing were added to the modifier-free molten metal: Mg in an amountof 4% by mass; R.E. (e.g. , misch metal) in an amount of 1.8%; and Sb inan amount of 0.0005% by mass. Here, the added Sb amount differed fromthat in Test Specimen No. 3-1.

(2) Measurement of Test Specimens

(i) In the same manner as in the case of Second Test, the analyzedcomposition and austenite proportion of each of the samples were found.These results are shown in Table 3A and Table 3B.

(ii) Samples were collected from each section of the respectiveaforementioned test specimens whose thickness was 25 mm, 12 mm, 5 mm and3 mm, and then they were measured for the spheroidized proportion ofgraphite, the number of graphite particles, and the hardness (Hv at 20kgf) in the same manner as Second Test.

(iii) The optical-microscope photographs of the respective samples areshown in FIG. 4A and FIG. 4B. #1 through #5 in the diagrams indicatethat the structural photographs show the samples' sections that wereprepared by dividing the samples into five sections evenly from the sanddie's upper-face side to the lower-face side. For example, #1 specifiesthe structure in the vicinity of the uppermost face, and #5 specifiesthe structure in the vicinity of the lowermost face. Note that thestructural photographs were taken after etching the samples' faces with3% nital.

(iv) In the same manner as in the case of First Test, the heat-resistantstrength of each of the samples was found, and was then shown in Table3B all together.

(3) Evaluation

(i) First of all, from the austenite proportions in Table 3B, it isunderstood that the bases' structure turned into an austenite phase inany one of the test specimens.

(ii) Next, as can be seen from FIG. 4A and FIG. 4B, it is understoodthat graphite crystallized spheroidally and virtually uniformly.

In particular, in the case of Test Specimen No. 3-2, the spheroidizedproportion exceeded 70% even when it is a 3-mm-thickness sample in whichthe molten metal was likely to be solidified rapidly. Moreover, evenwhen having any one of the thicknesses, the number of graphite particlesexceeded 200 pieces/mm², and furthermore the hardness could also bemaintained within a range of from 200 Hv to 300 Hv approximatelyregardless of the locations. From these, it is possible to say that theaustenitic cast iron (or cast product) according to the presentinvention excels in terms of mechanical characteristics, and moreoverexcels in terms of post-casting mechanical workabilities as well due tothe moderate hardness.

(iii) Of course, it is needless to say that anyone of these testspecimens had strength (or hardness) and heat-resistance strength thatwere equivalent to or more than those of conventional austenitic castirons (e.g., Reference Examples R3 and R4), as can be understood fromTable 3B, in the same manner as in the case of the above-described testspecimens, though the Ni contents are small.

Therefore, when using a cast iron like Test Specimen No. 3-2, it isappreciated that cast products with stable characteristics areobtainable, cast products which are not affected very much byconfigurations, not to mention in terms of the heat resistance, but interms of the other characteristics as well.

(Fourth Test)

(1) Manufacturing Method of Test Specimens

Although the compositions of the basic elements, and the types andaddition amounts of the auxiliary agents were changed, the others wereset in the same manner as those of Second Test and then twelve types oftest specimens (i.e., Test Specimen Nos. 4-1 through 4-12) weremanufactured.

Note that the addition of the auxiliary agents, such as an inoculantsagent and spheroidizing agents, was also carried out. The addedinoculant agent was “TOYOBARON BIL,” namely,74.18Si-1.23Ca-0.55Ba-0.72Bi-0.51Al—Fe, produced by TOYO DENKA Co., Ltd.This one was added in a proportion of 0.4% by mass with respect to themodifier-free molten metals. The addition of the spheroidizing agentswas carried out by adding the following to the modifier-free moltenmetals: an Mg simple substance in an amount of 4% by mass; R.E. (e.g. ,misch metal) in an amount of 1.8% by mass; and an Sb simple substance inan amount of 0.0005% by mass; with respect to the modifier-free moltenmetal being taken as 100%. Note that the amount of Mg was great becausethe disappearance and the like were considered.

Moreover, for the measurements of proof stress, tensile strength,elongation, reduction of area and Young's modulus, type-“A”

“Y”-shaped blocks as per JIS were prepared by means of mold casting, andthen φ6 round-bar test specimens were prepared from the rectangularvertical cross-sectional section of the resulting “Y”-shaped blocks.

(2) Measurement of Test Specimens

Twelve types of Test Specimen Nos. 4-1 through 4-12, which weremanufactured by means of the aforementioned manufacturing process butwhich had different blended compositions, were subjected to thefollowing analyses.

(i) In the same manner as in the case of First Test, the analyzedcomposition and austenite proportion of each of the samples were found.These results are shown in Table 4A and Table 4B.

(ii) Samples were collected from each section of the respectiveaforementioned test specimens whose thickness was 25 mm, 12 mm, 5 mm and3 mm, and then they were measured for the spheroidized proportion ofgraphite, the number of graphite particles, and the hardness (Hv at 20kgf) in the same manner as Second Test.

(iii) Samples were collected from a section of the respectiveaforementioned test specimens whose thickness was 25 mm, and theoptical-microscope photographs of the respective samples are shown inFIG. 6, respectively. Note that the structural photographs were takenafter etching the samples' face with 3% nital.

(iv) Further, in the same manner as First Test, the Ni_(eq) and Cr_(eq)were calculated from the analyzed composition of each of the entiresamples, and were then shown in Table 4B. Each of these Ni_(eq) andCr_(eq) was plotted with “▪” marks on the structural diagram in FIG. 2in a superimposed manner. The C_(s) was treated in the same manner as inthe case of First Test.

(v) In the measurements of proof stress, tensile strength, elongation,reduction of area and Young's modulus, tests were carried out at 800° C.in conformity to “G0567” as per JIS, and then those results are shown inTable 4B all together. Moreover, the measured data on conventional castirons are shown as Reference Example Nos. R3 through R6 in Table 4B alltogether.

(iv) The thermal-fatigue strength and thermal-fatigue life were measuredusing 0 5-mm round-bar test specimens that were collected frommold-casted type-“A” “Y”-shaped blocks as per JIS, and using φ8-mmround-bar test specimens that were collected from mold-casted type-“B”“Y”-shaped blocks as per JIS. In this test, while changing thetemperature of the test specimens with 100% constrained raterepetitively between 800° C. and 150° C., the test specimens wereexamined for the following: the number of cycles at which stress loweredby 10%; the number of cycles at which stress lowered by 25%; and thenumber of cycles at which they fractured apart. These results areillustrated in FIG. 21 (e.g., the results on the φ5-mm round-bar testspecimens) and FIG. 22 (e.g., φ8-mm round-bar test specimens),respectively. The “Stress Decline by 10%” and “Stress Decline by 25%”mean the number of cycles when peak stress on the tensile side decreasedby 10% from peak stress at the time of the number of cycle=2, and thenumber of cycles when it decreased by 25% therefrom, respectively.

(3) Evaluation

(i) First of all, in any one of the test specimens, it is understoodfrom the results of the X-ray analysis that the austenite proportionbecame 100% as shown in Table 4A, and it is understood from FIG. 6 thatno lamellar structures were seen in the Fe bases. Note however thatthere were even some test specimens in which structures that resembledlamellar structures seemed like to exist. However, no striped patternswere seen in those structures; when they were viewed in an enlargedmanner with a microscope, no long and thin rod-shaped structuralsubstances existed like those in lamellar structures but only structuralsubstances that were cut here and there existed. And, the structuralsubstances that are cut here and there do not become the cause of theoccurrence of cracks in austenite when they expand at the time of hightemperatures.

Moreover, concerning any one of the test specimens being labeled TestSpecimen Nos. 4-1 through 4-12 as well, a magnet did not react to their25-mm-thickness and 12-mm-thickness sections, and accordingly it wasascertained that they were free from magnetism. That is, being free frommagnetism means that ferrite, namely, a magnetic substance, does notexist, and consequently it is possible to speculate that they comprisedan austenite single phase.

Note that, regarding the 3-mm-thickness and 5-mm-thickness sections,there were some test specimens to which the magnet reacted. However,since it is not possible to think of that there are cases where ferriteexists and where no ferrite exists depending on thicknesses in anidentical test specimen, it is speculated that magnetism is exhibitedwith regard to the sections with thinner thickness, not because ferriteexists, but because carbides increase at the time of casting when thethickness gets thinner.

Moreover, it is apparent from Table 4B and FIG. 2 that a mathematicalformula, namely, Ni_(eq)≧Al·Cr_(eq)+B1, was satisfied in any one of thecases of Test Specimen Nos. 4-1 through 4-12 in which no lamellarstructures were seen in their Fe bases (that is, Test Specimen No. 4-9existed on a straight line with the least intercept, and this line isexpressed by Ni_(eq)=Al·Cr_(eq)+22.9).

On the contrary, it was ascertained that Niresist (FCDA-NiMn137 as perJIS) exhibited magnetism because the magnet reacted with respect to allof the 25-mm, 12-mm, 5-mm and 3-mm sections of the test specimens thatwere used in First Test. That is, since it exhibits magnetism, theexistence of ferrite, namely, a magnetic substance, is speculated.Moreover, as a result of calculation using “Ni_(eq)” and “Cr_(eq)” ofTest Specimen No. R2 being set forth in Table 1A, it was ascertained tobe Ni_(eq)<Al·Cr_(eq)+B1 (that is, Test Specimen No. R2 exists on astraight line, namely, Ni_(eq)=Al·Cr_(eq)+21.5).

Therefore, it is understood that, when defining “Ni_(eq)” and “Cr_(eq)”like the present specification and then considering the adaptability ofthe fourth and fifth conditions based on them, it is possible toaccurately demarcate whether a base's structure is an austenitic castiron (or cast product), which is made of austenite single phase, or not.

(ii) Next, as can be understood from Table 4A, Table 4B and FIG. 7, itis appreciated that Test Specimen Nos. 4-3, 4-4, 4-7, 4-8, 4-11 and 4-12whose Cu addition amounts were less comparatively had structuralconstructions and high-temperature strengths that were hardly inferiorto those of conventional austenitic cast irons (e.g., Reference ExampleNos. R3 and R4). Moreover, even when observing those test specimens withan optical microscope, no Cu peritectic structure was seen.

On the contrary, it is understood that, in Test Specimen Nos. 4-1, 4-2,4-5, 4-6, 4-9 and 4-10 whose Cu addition amounts were greatcomparatively, the elongation and reduction of area worsened at the timeof high temperature. When observing those test specimens with an opticalmicroscope, Cu peritectic structures were seen. Consequently, the causeof worsening the elongation and reduction of area at the time of hightemperature is speculated to be the resulting Cu peritectic structures.

Hence, it is understood that, when considering the adaptability of Culike the present specification, it is possible to accurately demarcatewhether it is an austenitic cast iron (or cast product) that is good interms of elongation and reduction of area or not.

(iii) Further, as can be seen from FIG. 8, it is understood that themore the Cr addition amount increased the higher the proof stress (MPa)became.

(iv) Therefore, it is appreciated that cast products, which are stable,not to mention, in the heat resistance but in the other characteristicsas well, are obtainable by using cast irons like Test Specimen Nos. 4-3,4-4, 4-7, 4-8, 4-11 and 4-12. Further, since the cast iron according toTest Specimen No. 4-3 not only comprised Ni in a lesser content but alsowas good in terms of the proof stress, it is possible to say that it wasthe best one among the aforementioned test specimens.

(v) Furthermore, as can be seen from FIG. 21 and FIG. 22, Test SpecimenNos. 4-3, 4-7, 4-8, 4-11 and 4-12, namely, the present austenitic castirons, had a thermal-fatigue life that was extended far greater thanthose of Test Specimen Nos. R5 and R6 and ferritic cast irons. Moreover,even when their thermal-fatigue lives were compared with those ofgeneral austenitic cast irons, the former was equivalent to or more thanthe latter.

In addition, it was ascertained from FIG. 21 and FIG. 22 that increasingthe Cr content even in the austenitic cast irons results in extendingthe thermal-fatigue life in any one of them. Likewise, it wasascertained from FIG. 21 that the increasing Cu content results inextending the thermal-fatigue life in any one of them even when their Crcontents are identical with each other.

(Fifth Test)

(1) Manufacturing Method of Test Specimens

Although the compositions of the basic elements, and the types andaddition amounts of the auxiliary agents were changed, the others wereset in the same manner as those of Fourth Test and then twelve types ofTest Specimen Nos. 5-1 through 5-12 were manufactured.

Note that the addition of the auxiliary agents, such as an inoculantagent and a spheroidizing agent, was also carried out. The addedinoculant agent was “TOYOBARON BIL,” namely,74.18Si-1.23Ca-0.55Ba-0.72Bi-0.51Al—Fe, produced by TOYO DENKA Co., Ltd.This one was added in a proportion of 0.4% by mass with respect to themodifier-free molten metals. For the spheroidizing agent, aspheroidizing agent that had the following in the following containedamounts was made use of: 4%-by-mass Mg simple substance; and1.8%-by-mass R.E. (e.g., misch metal); and the addition was carried outby adding it to the modifier-free molten metals so that the Mg residualamount became from 0.04 to 0.05% by mass with respect to the 100%modifier-free molten metals and the Sb-simple-substance residual amountbecame 0.0005% by mass with respect to them.

(2) Measurement of Test Specimens

Twelve types of Test Specimen Nos. 5-1 through 5-12, which weremanufactured by means of the aforementioned manufacturing process butwhich had different blended compositions, were subjected to thefollowing analyses.

(i) In the same manner as in the case of First Test, the analyzedcomposition and austenite proportion of each of the samples were found.These results are shown in Table 5A and Table 5B. Note that the analyzedcompositions in the present specification are based on wet analysis.

FIG. 9 illustrates an analyzed diagram (or XRD) in which samples thatwere collected from the 25-mm-thickness section of some of the testspecimens were subjected to an X-ray diffraction analysis. Moreover,FIG. 10 illustrates correlations between linear expansion coefficientsand temperatures that were measured for the other some of the testspecimens.

(ii) Samples were collected from each section of the respectiveaforementioned test specimens whose thickness was 25 mm, 12 mm, 5 mm and3 mm, and then they were measured for the spheroidized proportion ofgraphite, the number of graphite particles, and the hardness (Hv at 20kgf) in the same manner as Second Test. However, the subject of thespheroidized proportion of graphite, and that of the number of graphiteparticles were those whose graphite particle diameters were 5 μm ormore.

(iii) Using test specimens in which Cr, Mn, Ni and Cu were added in anamount of 1% bymass, respectively, and which had a thickness of 25 mm,12 mm, 5 mm and 3 mm, respectively, correlations between the values ofhardness rise and the plate thicknesses of those test specimens wereexamined when each of the respective elements was added independently.These results are illustrated in FIG. 23. Note that the composition of atest specimen, which made the basis for comparison (i.e., the datum forhardness), was Fe-3% C-4% Si.

(iv) Further, in the same manner as First Test, the Ni_(eq) and Cr_(eq)were calculated from the analyzed composition of each of the entiresamples, and were then shown in Table 4B. Each of these Ni_(eq) andCr_(eq) was plotted with “♦” marks on the structural diagram in FIG. 2in a superimposed manner. The C_(s) was treated in the same manner as inthe case of First Test.

(v) The oxidation resistance was evaluated by measuring the oxidizedweight reduction or oxidized weight increment based on “Z 2282” as perJIS. To be concrete, a variety of test specimens with φ20×20 mm, whichwere collected from type-“B” and type-“D” “Y”-shaped blocks as per JISthat were prepared by means of mold casting, were first retained in anair atmosphere at 800° C. for 100 hours. Iron balls whose shot sphericaldiameter was 0.4 mm were then projected to the test specimens that wereafter this heat treatment, and the projection was carried out untiloxide layers on their surfaces disappeared. Here, the oxidized weightincrement or oxidized weight decrement was each of the test specimens'mass increment or mass decrement per unit area. The oxidized weightincrement was obtained by deducting amass of each of the test specimensbefore the heat treatment from another mass of the test specimenimmediately after the aforementioned heat treatment (or before beingshot). The oxidized weight decrement was obtained by deducting a mass ofeach of the test specimens after being shot from another mass of thetest specimen immediately after the aforementioned heat treatment (orbefore being shot).

The thus found oxidized weight increments and oxidized weight decrementsare shown in Table 5B. Moreover, FIG. 11 illustrates the oxidized weightreductions of the respective test specimens with a bar graph. Note that,in FIG. 11, the oxidized weight reductions of some of the test specimensthat are shown in Tables 4A and 4B are also illustrated all together inaddition to the oxidized weight reductions of the test specimens thatare shown in Tables 5A and 5B.

Moreover, FIGS. 12( a) and (b), and FIGS. 13( a) and (b) illustrateresults of examining correlations between the contained amounts (oraddition amounts) of Ni, Mn, Cr and Cu (i.e., the basic elements thatare directed to the austenitic cast iron according to the presentinvention) and oxidized reductions on the basis of Fe-3% C-4% Si-“a”%Ni-“b”% Mn-“c”% Cr-“d”%Cu (% by mass).

(vi) The toughness was evaluated by carrying out a test based on “Z2242” as per JIS and then measuring the Charpy-impact values of therespective test specimens. To be concrete, the Charpy-impact values ofthe respective test specimens were measured under room temperature usingV-notched test specimens with 10×10×50 mm that were collected fromtype-“B” and type-“D” “Y”-shaped blocks as per JIS.

The thus found Charpy-impact values are shown in Table 5B. Moreover,FIG. 14 illustrates the Charpy-impact values of the respective testspecimens with a bar graph. Note that, in FIG. 14, the Charpy-impactvalues of some of the test specimens that are shown in Tables 4A and 4Bare also illustrated all together in addition to the Charpy-impactvalues of the test specimens that are shown in Tables 5A and 5B.

In addition, FIG. 15 illustrates correlations between the Charpy-impactvalues of the respective test specimens, which are shown in FIG. 14, andthe contained amounts of Cr in the respective test specimens.

(vii) In the measurements of proof stress, tensile strength, elongation,reduction of area and Young's modulus, tests were carried out at 800° C.in conformity to “G0567” as per JIS, and then those results are shown inTable 5B all together. Moreover, the measured data on conventional castirons are shown as reference examples (e.g., Nos. R3 through R6) inTable 5B all together.

Note that φ6-mm round-bar test specimens, which were collected fromtype-“A” “Y”-shaped blocks as per JIS that were prepared by means ofmold casting, were used for the measurements of proof stress, tensilestrength, elongation, reduction of area and Young's modulus.

FIG. 16 illustrates the 0.2% proof stress and fracture elongation ofeach of the test specimens with a bar graph. In this case as well, thoseof some of the test specimens that are shown in Tables 4A and 4B arealso illustrated all together in addition to those of the test specimensthat are shown in Tables 5A and 5B. Moreover, FIG. 17 illustratescorrelations between the respective test specimens' rupture elongationand their contained Cr amount or contained Cu amount.

Note that, other than Cr, the test specimens that are plotted in FIG. 17(b) had compositions as follows. One with Cu=0% had Ni=14.5% andMn=5.5%; another one with Cu=1.5% had Ni=13% and Mn=5.5%; still anotherone with Cu=3% had Ni=11.5% and Mn=5.5%; and the other one with Cu=4.5%had Ni=10.0% and Mn=5.5%.

Further, FIG. 18 illustrates the hardness (Hv at 20 kgf) of each of theabove-described test specimens with 5-mm plate thickness with a bargraph.

(viii) When each of the test specimens was cast, a molten-metal runningproperty was also investigated. To be concrete, an area of molten-metalrunning portion was found for a plate-configured test specimen that isillustrated in FIG. 19, area of molten-metal running portion which wasdetermined by subtracting an area of defective molten-metal runningportion from a total area of the test specimen that was obtained when amolten metal ran completely. Based on the resulting molten-metal runningproperty, the molten-metal running properties of the respective testspecimens were evaluated relatively.

FIG. 20 illustrates results of the relative evaluation on themolten-metal running properties with a bar graph. In the relativeevaluation, the area of the molten-metal running portion being exhibitedby Test Specimen Nos. 5-1, 5-9 and 4-3, that is, the test specimens thatwere considered showing the most satisfactory molten-metal runningproperty, was taken as “1,” and then the molten-metal running portionsof the other test specimens were evaluated relatively to that of theformers.

(ix) When each of the test specimens was cast, shrinkages wereinvestigated as well. To be concrete, as shown in FIG. 24, an innershrunk portion or outer shrunk portion, which occurred in a testspecimen, was filled up with shot balls with y50.5 mm, and then a totalweight of the filled shot balls was measured to evaluate a shrinkagemagnitude. FIG. 25 illustrates results of evaluating the shrinkagemagnitudes of the respective test specimens relatively while taking theshrinkage magnitude of Test Specimen No. R3 as “1.”

(x) First of all, correlations between heating-temperature ranges andlinear expansion coefficients were surveyed. The measurement of linearexpansion coefficient was carried out while changing the temperature ofa test specimen at an incremental temperature rate of 3° C. /min. withina specific range. This measurement was carried out in a nitrogenatmosphere with 0.05 MPa. A configuration of the used test specimens wasa squared-column shape with 3 mm×3 mm square and 15 mm in length. Thetest specimens had been annealed in advance by heating them to 950° C.or more in air. This measurement was carried out twice for each of thetest specimens, respectively, and then their averages were found. Theresulting outcomes are illustrated in FIG. 26. Note that, in FIG. 26,the designation, “E-06,” means 10⁻⁶ (i.e., parts per million).

Next, the heating-temperature width was limited to from 150 to 800° C.,and then an average linear expansion coefficient of each of the testspecimens was found. The resulting outcomes are illustrated in FIG. 27.

(xi) The respective test specimens' thermal conductivity was measured atroom temperature. The resulting outcomes are illustrated in FIG. 27.

(3) Evaluation

(i) First of all, in any one of the test specimens, it is understoodfrom the results of the X-ray analysis that the austenite proportionbecame 100% as shown in Table 5A. Moreover, this issue can also beascertained by comparing the graphic forms of Test Specimen No. R3,which has been known generally as an austenitic cast iron, or those ofTest Specimen No. R6, which has been known as a ferritic cast iron, withthose of Test Specimen No. 5-5, and the like, in the XRD diagram that isshown in FIG. 9 and the correlation diagram between temperatures andlinear expansion coefficients that is shown in FIG. 10. That is, it isseen from FIG. 9 that the XRD diagrams of Test Specimen No. 5-1, No. 5-5and No. 5-9 showed the same form as that of another Test Specimen No. R3comprising an austenitic phase, and that they showed different formsfrom that of Test Specimen No. R6 comprising a ferrite phase.

Moreover, the following can be seen from FIG. 10: Test Specimen No.5-5's correlation diagram between temperatures and linear expansioncoefficients showed a gentle form, which was similar to those of otherTest Specimen No. 4-3, No. R3 and No. R4 that comprised an austenitephase, up to around 910° C. at least; and the linear expansioncoefficient did not change abruptly unlike the linear expansioncoefficient of Test Specimen No. R6 comprising a ferrite phase that didso contrarily in a specific temperature zone (e.g., at around 750° C.).It was ascertained from these facts as well that the cast ironsaccording to Test Specimen Nos. 5-1 through 5-12 are austenitic castirons that virtually comprise an austenite single phase, respectively.

On the other hand, as can be seen from FIG. 2, such Test Specimen Nos.5-1 through 5-12 are positioned essentially in the mixture phase ofaustenite phase (A) and martensite phase (M) on the Schaeffler'sstructural diagram tentatively. However, regardless of the compositionswith such positioning, the austenitic cast iron according to the presentinvention turned into an austenite single phase virtually by adjustingan overall compositional range adequately.

Moreover, it is possible to speculate that the test specimens turn intoan austenite single phase even when the Ni equivalents are less in arange where the Cr equivalents falls in a range of from 7 to 9, becauseof the following facts: NiMn137 is not an austenite single phase atordinary temperature; and all of the test specimens (e.g., the testspecimens that are present below the dotted line in FIG. 2) turned intoan austenite phase, respectively, and such test specimens werespeculated to be less likely to turn into an austenite single phase thanis NiMn137 on the Schaeffler's structural diagram.

It is speculated that Cu and Ni are equivalent with respect to an Niequivalent in the Schaeffler's structural diagram; consequently, it ispossible to speculate that Test Specimen No. 5-12 keeps being anaustenite single phase even when the Cu content is increased from “0” to“1.5” and the Ni content is reduced from “8.5” to “7,” for instance,because there is not any change in the Ni equivalent. When doing thusly,it is possible to furthermore reduce the Ni content.

Moreover, Mn and Ni make a relationship, namely, 0.5:1, with respect toan Ni equivalent in the Schaeffler's structural diagram; consequently,it is possible to speculate that Test Specimen No. 5-12 keeps being anaustenite single phase even when the Mn content is reduced from “7.5” to“0.1” and the Ni content is increased from “8.5” to “12.2,” forinstance, because there is not any change in the Ni equivalent. Notethat, not increasing the amount of the Ni content alone, it ispermissible to increase both of the Ni content and Cu content. In thisway, when the Mn content, namely, a factor of raising hardness, can bedecreased, it is possible to lower the hardness of austenitic cast iron.

(ii) Next, it is appreciated from FIG. 11 that Test Specimen Nos. 5-1through 5-12 were good in terms of the oxidation resistance because theoxidized weight reduction was 100 mg/cm² or less in any one of them. Inparticular, as can be seen from FIG. 12 and FIG. 13, the oxidized weightreduction is affected greatly by the contained elements' types and theircontained amounts, and their influential powers become the followingorder: Cr>Ni>Cu>Mn. In austenitic cast irons like the present inventionin which the Ni contents are made less considerably than those ofconventional ones, it was ascertained that having them contain Cr or Cu(Cr especially) is effective in the improvement of their oxidationresistance.

(iii) On the other hand, it is seen from FIG. 14 and FIG. 15 that thetoughness of the austenitic cast irons declined as the contained amountsof Cr increased. However, it was ascertained that those whose containedamount of Cr was 2.5% by mass approximately had toughness that isequivalent to or more than those of conventional austenitic cast iron(i.e., Test Specimen No. R5) and ferritic cast iron (i.e., Test SpecimenNo. R6). Further, it is also seen from FIG. 15 that there was such atendency that the less the contained amount of Mn was the higher thetoughness (or Charpy-impact value) of the austenitic cast irons became.

From FIG. 12 and FIG. 15, it is possible to say that it is morepreferable that the contained amount of Cr can be from 0.5 to 2% bymass, or further from 0.5 to 1.5% by mass approximately, in order tosecure the oxidation resistance and toughness that can be equivalent toor more than those of conventional austenitic cast irons (e.g., TestSpecimen Nos. R3 and R4).

(iv) It is seen from FIG. 16 that any one of the cast irons according toTest Specimen Nos. 5-1 through 5-12 had high-temperature strength (e.g.,0.2%proof stress and fracture elongation at 800° C.) that was the sameor more than those of conventional austenitic cast irons (e.g., TestSpecimen Nos. R3 through R5) and ferritic cast iron (e.g., Test SpecimenNo. R6).

Moreover, it is seen from FIG. 17( a) that, though the austenitic castirons' fracture elongation at high temperature was improved bymeans ofincreasing the contained amount of Cr, it became a virtually saturatedstate when that contained amount was 2.5% by mass approximately. On theother hand, it is seen from FIG. 17( b) that the austenitic cast irons'fracture elongation at high temperature was decreased sharply by meansof increasing the contained amount of Cu. Hence, it is possible to saythat it is preferable that the upper limit of the contained amount of Crcan be 3% by mass or less, or further 2.5% by mass, approximately; andit is preferable that the upper limit of the contained amount of Cu canbe 2% by mass approximately.

(v) From FIG. 18, it seems that Test Specimen Nos. 5-1 through 5-12(that is, their sections with 5-mm plate thickness) also exhibitedfavorable workability in cutting, and the like, because the hardness ofany one of them was 250 Hv approximately.

Note that, as can be seen from FIG. 23, the hardness of test specimen isaffected by the additive elements and plate thicknesses. That is, itbecomes such a tendency that the hardness of test specimen rises whenadding Cr or Mn. On the contrary, it becomes such an opposite tendencythat the hardness of test specimen declines when adding Ni or Cu. Fromthese facts, it is appreciated that an austenitic cast iron with desiredhardness is obtainable by means of the selection of these additiveelements and the adjustment of their addition amounts.

However, the resulting hardness is affected by the thickness of testspecimen (or cast product) as well. Although the influence of theadditive elements is great at sections with smaller plate thicknesses,it was appreciated that the greater those plate thicknesses become thesmaller the influence of any one of the additive elements becomes andthen the hardness shows such a tendency that it converges to that of atest specimen comprising a datum composition.

(vi) From the relative evaluation on molten-metal running that isillustrated in FIG. 20, any one of Test Specimen Nos. 5-1 through 5-12was superior to a conventional austenitic cast iron (e.g., Test SpecimenNo. R5) in terms of the molten-metal running property. In particular, itwas also ascertained that the austenitic cast irons, which are directedto the present invention, were superior to another conventionalaustenitic cast iron (e.g., Test Specimen No. R3) in terms of themolten-metal running property, because their molten-metal runningproperties were very favorable, that is, they were about 1 in all ofthem, excepting Test Specimen No. 5-11, regardless of being evaluatedrelatively.

(vii) From the relative evaluation on shrinkage magnitude that isillustrated in FIG. 25, it was seen that, in any one of the testspecimens, the shrinkage magnitude was less than that in arepresentative austenitic cast iron (e.g., Test Specimen No. R3). To beconcrete, it was from 70 to 85% approximately in the test specimens thatexhibited the greater shrinkage magnitudes; and it was from 35 to 50%approximately in the test specimens that exhibited the lesser shrinkagemagnitudes; and accordingly the shrinkage magnitudes could beapproximated to the shrinkage magnitude of a ferritic cast iron (e.g.,Test Specimen No. R5).

(viii) It was seen from FIG. 26 that, regardless of heating-temperaturezones, the average linear expansion coefficients of the austenitic castirons according to Test Specimen No. 4-3 and No. 5-5 were virtuallyequal to the average linear expansion coefficient of an existingaustenitic cast iron (e.g., Test Specimen No. R4).

Moreover, according to FIG. 27, not only the average linear expansioncoefficients of Test Specimen Nos. 5-1 through 5-12 as well as those ofTest Specimen Nos. 4-3, 4-4, 4-11 and 4-12 were surely higher than thoseof existing ferritic cast irons (e.g., Test Specimen Nos. R5 and R6),but also they were a little bit higher than those of existing austeniticcast irons (e.g., Test Specimen Nos. R3 and R4) roughly.

(ix) According to FIG. 28, although the thermal conductivities of TestSpecimen Nos. 5-1 through 5-12 as well as those of Test Specimen Nos.4-3, 4-4, 4-11 and 4-12 were lower than the thermal conductivities ofexisting ferritic cast irons (e.g., Test Specimen Nos. R5 and R6), theywere virtually equal to the thermal conductivity of an existingaustenitic cast iron (e.g., Test Specimen No. R3).

(x) It is possible to say that Test Specimen Nos. 5-5 and 5-6 wereexcellent materials, because they exhibited moderate hardness and weregood in terms of the oxidation resistance despite their contents of Nithat were less.

(Sixth Test)

(1) Manufacturing Method of Test Specimens

Although the compositions of the basic elements, and the types andaddition amounts of the auxiliary agents were changed, the others wereset in the same manner as those of Second Test and then six types ofTest Specimen Nos. 6-1 through 6-6 were manufactured. Note that theaddition of the auxiliary agents, such as an ioculant agent and aspheroidizing agent, was also carried out when casting the respectivetest specimens.

The added inoculants agent was “TOYOBARON BIL,” namely,74.18Si-1.23Ca-0.55Ba-0.72Bi-0.51Al—Fe, produced by TOYO DENKA Co., Ltd.This one was added in a proportion of 0.4% by mass with respect to themodifier-free molten metals.

For the spheroidizing agent, a spheroidizing agent, which had an Mgsimple substance and R.E. (e.g., misch metal) in a contained amount of4% by mass and 1.8% by mass respectively, was made use of; and theaddition was carried out to the stock molten metals so that a residualamount of Mg became from 0.04 to 0.06% by mass and a residual amount ofSb simple substance became 0.0005% by mass with respect to the 100%stock molten metals.

(2) Measurement of Test Specimens

Six types of Test Specimen Nos. 6-1 through 6-6, which were manufacturedby means of the aforementioned manufacturing process but which haddifferent blended compositions, were subjected to the followinganalyses.

(i) In the same manner as in the case of First Test, the following werefound, respectively: the analyzed composition of each of the samples;C_(eq), Ni_(eq) and Cr_(eq) based on that analyzed composition; and theaustenite proportion. These results are shown in Table 6A. Note that,although the analysis on the major elements of the respective testspecimens was carried out based on wet analysis, a gas analysis was alsocarried out with respect to the respective test specimens in conjunctionwith the former analysis. In this gas-analysis method, gases that weregasified by means of high-frequency combustion, were quantified byinfrared absorption spectrophotometry using an analyzing apparatus thatwas produced by LECO Corporation, and thereby 0 was quantified byinfrared absorption spectrophotometry and N was quantified by thermalconductivity method.

(ii) Samples were collected from each section of the respectiveaforementioned test specimens whose thickness was 25 mm, 12 mm, 5 mm and3 mm, and then they were measured for the spheroidized proportion ofgraphite, the number of graphite particles, and the hardness (Hv at 20kgf) in the same manner as Second Test. However, the subject of thespheroidized proportion of graphite, and that of the number of graphiteparticles were those whose graphite particle diameters were 5 μm ormore. These results are shown in Table 6B.

(iii) The oxidation resistance of each of the test specimens wasevaluated by measuring the oxidized weight reduction or oxidized weightincrement based on “Z 2282” as per JIS. To be concrete, a variety oftest specimens with 20×30×5 mm, which were collected from type-“B” andtype-“D” “Y”-shaped blocks as per JIS that were prepared by means ofmold casting, were first retained in each of 750-° C., 800-° C. and850-° C. air atmospheres for 100 hours. Iron balls whose shot sphericaldiameter was 0.4 mm were then projected to the test specimens that wereafter this heat treatment, and the projection was carried out untiloxide layers on their surfaces disappeared. Here, the oxidized weightincrement or oxidized weight decrement was each of the test specimens'mass increment or mass decrement per unit area. The oxidized weightincrement was obtained by deducting amass of each of the test specimensbefore the heat treatment from another mass of the test specimenimmediately after the aforementioned heat treatment (or before beingshot). The oxidized weight decrement was a value that was obtained bydividing a deducted value, which was obtained by deducting a mass ofeach of the test specimens after being shot from another mass of thetest specimen immediately after the aforementioned heat treatment (orbefore being shot), with a surface area of the test specimen. The thusfound oxidized weight increments and oxidized weight decrements areshown in Table 6B.

Furthermore, regarding the cases as well where the temperature in airatmosphere in which the respective test specimens were heated and thenretained were set at 750° C. and 850° C. instead of 800° C., theoxidized weight reduction and oxidized weight increment were measuredfor each of the test specimens. Not only the results are shown in Table6C but also the oxidized weight reductions of the respective testspecimens are illustrated with a bar graph in FIG. 29. Note that theoxidized weight reductions of the Test Specimen Nos. R3, R4, R5 and R7that comprised conventional cast irons are also shown all together inTable 6A, Table 6B and FIG. 29 for comparison, in addition to those ofpresent Test Specimen Nos. 6-1 through 6-6. Incidentally, the oxidizedweight reductions that are given in Table 6A, Table 6B and FIG. 29 areaveraged values of their twice-measured values, and the oxidized weightincrements are averaged values of their thrice-measured values.

(vi) The measurements of proof stress (e.g., 0.2% proof stress), tensilestrength and elongation were carried out onto the respective testspecimens whose temperature was 800° C. in conformity to “G0567” as perJIS. For these measurements, φ6-mm round-bar test specimens, which werecollected from type-“B” “Y”-shaped blocks as per JIS that were preparedby means of mold casting, were used. Those results are shown in Table 6Ball together.

Furthermore, regarding the cases as well where the temperature of eachof the test specimens was set at room temperature (or R. T.), 750° C.and 850° C. in addition to 800° C., the proof stress, tensile strengthand elongation were measured similarly for each of the test specimens.Not only the results are shown in Table 6C but also the proof stresses,tensile strengths and elongations of the respective test specimens areillustrated with a bar graph in FIGS. 30 through 32, respectively. Inthese cases as well, the proof stresses, tensile strengths andelongations of the Test Specimen Nos. R3, R4, R5 and R7 that comprisedconventional cast irons are also shown all together for comparison, inaddition to those of present Test Specimens Nos. 6-1 through 6-6.Incidentally, the proof stresses, tensile strengths and elongations thatare given in Table 6A, Table 6B and FIGS. 30 through 32 are averagedvalues of their thrice-measured values.

(v) A thermal-fatigue life of each of the test specimens was measured.The measurement of this thermal-fatigue life was carried out in thefollowing manner. Test specimens, namely, φ8-mm round bars, which werecollected from type-“B” “Y”-shaped blocks as per JIS and which comprisedvarious compositions, were made ready.

While setting a constrained ratio to each of the test specimens at 100%,those test specimens' temperature was fluctuated between 800° C. and200° C. repeatedly to examine the following respectively: the number ofcycles at which stresses that acted on the test specimens lowered by10%; the number of cycles at which they lowered by 25%; the number ofcycles at which they lowered by 50%; and the number of cycles at whichthe test specimens fractured apart. Furthermore, in addition to the casewhere a constrained ratio to each of the test specimens was set at 100%,regarding the cases as well where the constrained ratio was set at 50%and 30%, each of the following was found similarly: the number of cyclesat which stresses that acted on the test specimens lowered by 10%, 25%and 50%; and the number of cycles at which the test specimens fracturedapart.

Note that this thermal fatigue test was carried out with a coffin-typethermal fatigue testing machine, and that the constrained ratio η meansa proportion of a constrained magnitude “B” with respect to a freeexpansion magnitude “A” (i.e., η=“B”/“A”×100 (o)). Moreover, “10%-stressdecline,” “25%-stress decline,” or “50%-stress decline” means thefollowing: the number of cycles when a peak stress on the tensile sidedecreased by 10% therefrom; the number of cycles when a peak stress onthe tensile side decreased by 25% therefrom; and the number of cycleswhen a peak stress on the tensile side decreased by 50% therefrom,respectively; on the basis of a peak stress when the number of cycles=2.

In addition to the aforementioned thermal-fatigue test, each of the testspecimens' temperature was fluctuated between 150° C. and 800° C.repeatedly while setting the constrained ratio of those test specimensat 100%, thereby examining the number of cycles at which stresses thatacted on the test specimens lowered by 10%, the number of cycles atwhich they lowered by 25%, and the number of cycles at which the testspecimens fractured apart, respectively.

Not only these results are shown in Table 6C collectively but also thethermal-fatigue lives of the test specimens are illustrated with a bargraph in FIG. 33. Note that, regarding Test Specimen Nos. R3, R4, R5 andR7 as well that comprised conventional cast irons, each of theirthermal-fatigue lives is shown all together for comparison, in additionto those of present Test Specimen Nos. 6-1 through 6-6.

(vi) A linear expansion coefficient of each of the test specimens wasfound. This linear expansion coefficient was found by measuring a changeof each of the test specimens in the length when the test specimens'temperature was changed from 40° C. and up to 900° C. at atemperature-increment rate of 3° C/min in the presence of nitrogenatmosphere (e.g., 0.05 MPa). A configuration of the test specimens thatwere used for this measurement was adapted into a squared-column shapewith 3 mm×3 mm squared section and 15 mm in length. The respective testspecimens had been annealed in advance by heating them to 950° C. ormore in air. These results are given in Table 6C.

Note that, in Table 6C, being set forth as “‘Averaged’ Linear ExpansionCoefficient” means average thermal expansion coefficients from 40 and upto 900° C., and that these averaged linear expansion coefficients arevalues that were obtained by further averaging their twice-measuredvalues (averaged linear expansion coefficients) being found for therespective test specimens.

(3) Evaluation

(i) First of all, in any one of the Test Specimen Nos. 6-1 through 6-6in Table 6A, the austenite proportion was 100% virtually according tothe results of the X-ray analysis. This issue can also be understoodfrom the fact that those linear expansion coefficients of Test SpecimenNos. 6-1 through 6-6 were equivalent to that of Test Specimen No. R3that has been known generally as an austenitic cast iron.

(ii) Next, as can be understood from Table 6B, the spheroids zedproportions in the respective test specimens were high regardless oftheir plate thicknesses, and the number of graphite particles becamesufficient even in test specimens with larger plate thicknesses. Thatis, it is understood that graphite crystallized as spherical shapesvirtually evenly in any one of Test Specimen Nos. 6-1 through 6-6regardless of the plate thickness. Therefore, when having the samecompositions as those of these test specimens, cast products can beobtained, cast products which comprise metallic structures in whichgraphite is crystallized as spherical shapes virtually evenly not onlyin their surfaces but also in their insides.

Further, in the test specimen with any one of the plate thicknesses, itis possible to say that the austenitic cast irons (or cast products)according to the present example was good not only in terms ofmechanical characteristics but also in terms of machinability, becausethe hardness was stabilized as from 200 Hv to 300 Hv approximately.

(iii) As can be appreciated from Table 6B, Table 6C and FIG. 29, theoxidized weight reduction was 30 mg/cm² or less approximately when theheating temperature was 750° C.; and it was as small as 50 mg/cm² orless approximately when the heating temperature was 800° C.; in any oneof Test Specimen Nos. 6-1 through 6-6. It is understood that theaustenitic cast irons according to the present example were good interms of the oxidation resistance, because any one of the oxidizedweight reductions was 100 mg/cm² or less approximately even in the casewhere the heating temperature was 850° C.

However, it is understood that the Cr content or the Ni content affectsgreatly the suppression of the oxidized weight reduction, that is, theimprovement of the oxidation resistance, when comparing Test SpecimenNo. 6-1 with Test Specimen No. 6-5 or comparing Test Specimen No. 6-5with Test Specimen No. 6-6, for instance. In particular, in TestSpecimen No. 6-3 in which both of the Cr content and Ni content weregreat, it was ascertained that the suppression of the oxidized weightreduction was so remarkable as being at the same level as that in TestSpecimen No. R7.

(iv) As can be appreciated from Table 6B, Table 6C and FIGS. 30 through32, the proof stress, tensile strength and fracture elongation wereequivalent to or more than those of Test Specimen No. R3 or TestSpecimen No. R4, namely, those of conventional austenitic cast irons, inany one of Test Specimen Nos. 6-1 through 6-6. Especially, in TestSpecimen No. 6-2 that did not include any Cu, the proof stress andtensile strength hardly lowered, and the elongation improved remarkablyto exhibit high ductility.

In particular, in Test Specimen No. 6-6, the oxidation resistanceimproved considerably because it contained Cr. Besides, it demonstratedsuch a good characteristic that the hardness was low comparativelybecause the contained amount of Cr was 1.5% in Test Specimen No. 6-6 andwas less than 2.5% in Test Specimen No. 6-3.

(v) As can be appreciated from Table 6C and FIG. 33, any one of thethermal-fatigue lives of Test Specimen Nos. 6-1 through 6-6 wereequivalent to or more than those of Test Specimen No. R3 or TestSpecimen No. R4, namely, those of general austenitic cast irons.However, when viewing the thermal-fatigue life as a whole, the greaterthe Ni content was and the less the Cr content was the longer the testspecimens' thermal-fatigue life became. Moreover, the test specimensthat contained an appropriate amount of Cu had a longer thermal-fatiguelife rather than those that did not.

TABLE 1A Entire Cast Iron Test Blended (or Target) Analyzed CompositionSpecimen Composition (% by mass) (% by mass) No. C Si Cr Ni Mn Cu (Mo) CSi Cr Ni Mn Cu C_(eq) 1-1 3 2.3 0 13 6.5 6.5 3.28 2.11 0.034 13.6 6.516.49 4.0 1-2 3 4 0 13 6.5 6.5 2.88 4.38 0.009 13.53 6.79 6.4 4.3 1-3 3 42 13 6.5 6.5 3.09 4.31 1.98 13.75 6.71 6.43 4.5 1-4 3 4 2 9 6.5 13 2.894.17 2.07 9.71 6.76 11.91 4.3 1-5 3 4 2 6 13 13 3.15 4.42 2.10 6.3013.35 10.61 4.6 R1 3 2.25 2 15 — 6.5 3.1 2.0 3.5 16.04 1.10 6.77 3.7 R23 2.25 — 13 7 — 3.0 2.4 0.5 13.52 7.35 0.022 3.8 R3 2.0 5.0 2 35 0.6 —2.0 4.7 2.0 35.03 0.56 — 3.5 R4 2.8 2.8 2 20.5 0.8 — 2.3 2.8 2.0 21.221.21 — 3.3 R5 3.8 2.6 — — 0.8 or — 3.5 2.7 — — 0.44 — 4.4 less R6 3.14.2 — — 0.6 or Mo 2.8 4.4 — — 0.40 — 4.2 less 0.5 Test Fe Base SpecimenAnalyzed Composition (% by mass) No. C_(s) Si Cr Ni Mn Cu Ni_(eq)Cr_(eq) A.P. (%) *1 Remarks 1-1 Not 2.3 0.1 10.4 6.5 7.2 21.9 3.6 100Analyzable 1-2 Not 4.6 0 9.4 5.8 7.1 20.4 6.9 100 Analyzable 1-3 Not 4.51.5 10.1 5.6 6.7 20.6 8.3 100 Analyzable 1-4 Not 5.1 1.6 7.5 7 6.6 18.69.3 50 Analyzable 1-5 Not 4.5 1.6 4.2 14.5 5.3 17.8 8.4 30 Analyzable R1Unanalyzed 24.4 6.4 100 *2 R2 Unanalyzed 18.2 4.1 100 JIS: FCDA-NiMn137R3 Unanalyzed 36.3 8.9 100 JIS: FCDA-NiSiCr3522 (or ASTM: D-5S) R4Unanalyzed 22.8 6.1 100 JIS: FCDA-NiCr202 (or ASTM: D-2) R5 Unanalyzed1.2 4.1 0 JIS: FCDA 450 R6 Unanalyzed 1.2 6.7 0 HiSiMo FCD (Note) *1:“A.P.” stands for Austenite Proportion. *2: JIS: FCA-NiCuCr1562 (one inwhich graphite was spheroidized by means of spheroidizing treatment)

TABLE 1B Heat-resistance Strength (MPa) Test 150° C. 800° C. SpecimenProof Tensile Proof Tensile No. Stress Strength Stress Strength Remarks1-1 Unmeasured 1-2 Unmeasured 1-3 Unmeasured 1-4 Unmeasured 1-5Unmeasured R1 Unmeasured JIS: FCA-NiCuCr1562 (one in which graphite wasspheroidized by means of spheroidizing treatment) R2 Unmeasured JIS:FCDA-NiMn137 R3 181 419 73 116 JIS: FCDA-NiSiMn3522 (or ASTM: D-5S) R4219 446 87 134 JIS: FCDA-NiCr202 (or ASTM: D-2) R5 403 503 27 48 JIS:FCD 450 R6 459 610 28 43 HiSiMo FCD (or TS: FCDA4)

TABLE 2A Blended Composition Test of Basic Elements Analyzed Compositionof Specimen (% by mass) Entire Cast Iron (% by mass) No. C Si Cr Ni MnCu C Si Cr Ni Mn Cu Mg Ce S P 2-1 3.0 4.0 1.5 10.0 5.5 6.0 2.95 3.901.41 10.10 5.52 6.18 0.04 0.01 0.005 0.03 2-2 3.0 4.0 1.5 10.0 5.5 6.53.04 3.75 1.41 9.90 5.47 7.39 0.04 0.01 0.007 0.03 2-3 3.0 4.0 1.5 6.014.5 6.0 3.01 4.04 1.52 6.26 14.50 6.09 0.03 0.01 0.001 0.04 2-4 3.0 4.01.5 10.0 5.5 5.5 3.05 3.95 1.54 10.00 5.42 5.53 0.04 0.01 0.008 0.03 2-53.0 4.0 1.5 7.5 14.5 6.0 3.06 4.19 1.48 7.75 14.5 6.05 0.04 0.01 0.0020.04 2-6 3.0 4.0 1.5 7.5 7.5 6.0 3.04 3.81 1.51 7.42 7.52 6.03 0.03 0.010.006 0.03 2-7 3.0 4.0 1.5 7.5 10.0 6.0 3.04 3.98 1.50 7.64 10.10 5.930.03 0.01 0.005 0.03 2-8 3.0 4.0 1.5 6.0 10.0 6.0 3.02 4.00 1.52 6.0610.20 5.91 0.03 0.01 0.004 0.03 2-9 3.0 4.0 1.5 5.0 14.5 8.0 3.04 3.971.48 5.22 14.90 8.48 0.04 0.01 0.001 0.03 2-10 3.0 4.0 1.5 7.5 7.5 8.02.95 3.85 1.47 7.56 7.67 7.91 0.03 0.01 0.005 0.03 2-11 3.0 4.0 1.5 7.55.5 8.0 3.00 2.96 1.52 7.66 5.74 8.28 0.01 — 0.005 0.03 2-12 3.0 4.0 1.56.0 10.0 8.0 3.04 3.94 1.49 6.10 9.84 7.61 0.03 0.01 0.003 0.03 2-13 3.04.0 1.5 6.0 7.5 8.0 3.01 3.87 1.51 6.31 7.73 7.86 0.03 0.01 0.003 0.03

TABLE 2B Cast Structure Number of Spheroidized Particles Proportion (%)(Pieces/mm²) Heat-resistant Strength (MPa) (12-mm Section) Hardness 150°C. 800° C. T.S. Equivalent (%) Austenite (10-μm Minimum Hv (20 kgf)Proof Tensile Proof Tensile No. C_(eq) Ni_(eq) Cr_(eq) Proportion (%)Graphite Particle Dia.) (12-mm Section) Stress Strength Stress Strength2-1 4.25 19.9 7.3 100 80 226 217 240 412 91 93 2-2 4.29 20.9 7.0 100 75108 231 244 374 87 88 2-3 4.36 20.5 7.6 100 74 51 459 — — — — 2-4 4.3719.1 7.5 100 71 244 209 239 417 95 97 2-5 4.46 22.0 7.8 100 66 71 487 —— — — 2-6 4.31 18.1 7.2 100 50 183 269 241 301 84 87 2-7 4.37 19.5 7.5 —48 180 266 — — — — 2-8 4.35 18.0 7.5 100 41 174 332 260 320 93 113  2-94.36 22.1 7.4 — Chilled No Graphite 538 — — — — 2-10 4.23 20.2 7.2 —Chilled No Graphite 467 — — — — 2-11 3.99 19.7 6.0 — Chilled No Graphite363 — — — — 2-12 4.35 19.5 7.4 — Chilled No Graphite 546 — — — — 2-134.30 18.9 7.3 — Chilled No Graphite 490 — — — —

TABLE 3A Blended Composition of Basic Elements T.S. (% by mass) AnalyzedComposition (% by mass) No. C Si Cr Ni Mn Cu C Si Cr Ni Mn Cu Mg Ce BaAl Ca S P 3-1 3 4 1.5 10 5.6 6.7 2.95 4.24 1.45 9.77 5.37 6.48 0.03 0.010.0011 0.01 0.01 0.004 0 3-2 3 4 1.5 10 5.5 5.5 3.11 3.67 1.5 10.4 5.655.67 0.03 0.01 0.002 0.002 0 0.007 0.04

TABLE 3B Spheroidized Proportion Number of Particles (10-μm Mini. (10-μmMini. Graphite Graphite Particle Dia.) Particle Dia.) EquivalentAustenite Thickness of Test Specimen T.S. (%) Proportion at CollectedSections (mm) No. C_(eq) Ni_(eq) Cr_(eq) (%) 25 12 5 3 25 12 5 3 3-14.33 20.0 7.8 100 72.8 64.5 57 60 99.2 74.9 63.5 283 3-2 4.33 19.9 7.0100 85 83 74 71 233 241 387 282 Hardness Hv (20 kgf) Thickness of TestHeat-resistant Specimen at Collected Strength (MPa) T.S. Sections (mm)150° C. 800° C. No. 25 12 5 3 P.S. *1 T.S. *2 P.S. *1 T.S. *2 3-1 213221 221 346 239 380 86 87 3-2 218 213 227 303 246 405 96 98 (Note) *1:“P.S.” stands for Proof Stress. *2: “T.S.” stands for Tensile Stress.

TABLE 4 Blended Composition of Basic Elements Analyzed Composition ofT.S. (% by mass) Entire Cast Product (% by mass) No. C Si Cr Mn Ni Cu(Mo) C Si Cr Ni Mn Cu (Mo) 4-1 3.0 4.0 1.5 5.5 10.0 4.5 2.95 3.88 1.5810.07 5.79 4.54 4-2 3.0 4.0 1.5 5.5 11.5 3.0 3.01 3.91 1.62 11.76 5.83.09 4-3 3.0 4.0 1.5 5.5 13.0 1.5 2.96 3.87 1.64 12.94 5.98 1.56 4-4 3.04.0 1.5 5.5 14.5 0.0 3.04 3.95 1.62 14.11 6.02 0.01 4-5 3.0 4.0 0.5 5.510.0 4.5 3.01 3.95 0.56 9.85 5.85 4.63 4-6 3.0 4.0 0.5 5.5 11.5 3.0 2.974 0.55 11.61 5.5 3.12 4-7 3.0 4.0 0.5 5.5 13.0 1.5 2.93 3.9 0.55 13.115.95 1.57 4-8 3.0 4.0 0.5 5.5 14.5 0.0 2.99 4.08 0.55 14.58 6.03 0 4-93.0 4.0 0.0 5.5 10.0 4.5 2.92 3.89 0.01 10.17 5.4 4.56 4-10 3.0 4.0 0.05.5 11.5 3.0 2.92 3.85 0.01 11.76 5.73 3.1 4-11 3.0 4.0 0.0 5.5 13.0 1.52.96 3.85 0.01 13.23 5.84 1.54 4-12 3.0 4.0 0.0 5.5 14.5 0.0 2.99 3.880.01 14.58 5.98 0 R3 2.0 5.0 2.0 0.6 35.0 — 2.0 4.7 2.0 35.0 0.6 — R42.8 2.8 2.0 0.8 20.5 — 2.3 2.8 2.0 21.2 1.2 — R5 3.8 2.8 — 0.8 — — 3.52.7 — — 0.4 — or less R6 3.1 4.2 — 0.6 — Mo 2.8 4.4 — — 0.4 Mo or 0.50.47 less Analyzed Composition of Entire Cast Product T.S. (% by mass)Equivalent (%) A.P. No. P Mg Ce S C_(eq) Ni_(eq) Cr_(eq) (%) *1 Remarks4-1 0.018 0.038 0.015 0.006 4.2 18.4 7.4 100 4-2 0.021 0.039 0.016 0.0054.3 18.7 7.5 100 4-3 0.022 0.034 0.015 0.004 4.3 18.4 7.4 100 4-4 0.0210.039 0.017 0.005 4.4 18.0 7.5 100 4-5 0.021 0.043 0.017 0.005 4.3 18.36.5 100 4-6 0.019 0.041 0.017 0.006 4.3 18.4 6.6 100 4-7 0.023 0.0410.018 0.005 4.2 18.6 6.4 100 4-8 0.022 0.042 0.019 0.006 4.4 18.5 6.7100 4-9 0.024 0.049 0.02 0.005 4.2 18.3 5.8 100 4-10 0.024 0.04 0.0180.004 4.2 18.6 5.8 100 4-11 0.024 0.039 0.018 0.005 4.2 18.6 5.8 1004-12 0.022 0.04 0.0019 0.006 4.3 18.5 5.8 100 R3 0.017 0.066 — 0.010 3.536.3 8.9 100 JIS: FCDA-NiSiCr3522 (or ASTM: D-5S) R4 0.022 0.052 — 0.0183.3 22.8 6.1 100 JIS: FCDA-NiCr202 (or ASTM: D-2) R5 0.040 0.028 — 0.0104.4 1.2 4.1 0 JIS: FCD450 R6 0.038 0.036 — 0.009 4.2 1.2 6.7 0 HiSiMoFCD (or TSFCDA4) (Note) *1: “A.P.” stands for Austenite Proportion.

TABLE 4B Cast Structure Spheroidized Proportion (%) Number of Particles(Graphite's Min. (pieces/mm²) Particle (Graphite's Min. HardnessHeat-resistant Strength (800° C.) Dia.: 10 μm) Particle Dia.: 10 μm) Hv(20 kgf) Proof Tensile Reduction Young's T.S. Thickness of Test Specimenat Collected Section (mm) Stress Stress Elongation of Area Modulus No.25 12 5 3 25 12 5 3 25 12 5 3 MPa MPa % % GPa Remarks 4-1 84 78 79 63158 315 328 177 221 219 244 274 97 100 2 0 84 4-2 89 87 90 82 228 320462 257 203 206 230 257 87 120 3 1 75 4-3 82 90 88 79 104 265 457 360192 192 211 239 75 121 19 17 82 4-4 80 87 86 83 205 294 372 385 188 190214 248 78 126 31 29 77 4-5 73 71 63 59 150 417 180 112 201 194 205 215— 79 0 0 78 4-6 88 85 87 87 124 240 416 553 189 181 192 207 79 82 2 1 714-7 81 88 83 82 197 256 276 245 179 167 177 198 71 110 13 13 83 4-8 7982 90 86 191 204 481 417 180 167 180 205 73 114 29 29 81 4-9 86 77 74 63142 256 152 152 189 190 201 209 — 57 0 0 74 4-10 69 86 85 80 146 273 298241 185 178 185 200 73 75 1 0 71 4-11 77 83 81 85 274 286 322 409 178163 170 193 68 105 12 9 78 4-12 77 85 78 82 226 221 243 320 165 163 172191 71 111 29 26 80 R3 Unmeasured 73 116 31 32 101 JIS: FCDA- NiSiCr3552(or ASTM: D-5S) R4 Unmeasured 87 134 25 23 90 JIS: FCDA- NiCr202 (orASTM: D2) R5 Unmeasured 27 48 35 35 41 JIS: FCD450 R6 Unmeasured 28 4372 68 65 HiSiMo FCD (or TSFCDA4)

TABLE 5A Blended Composition of Basic Element Analyzed Composition ofT.S. (% by mass) Entire Cast Product (% by mass) No. C Si Cr Mn Ni Cu(Mo) C Si Cr Ni Mn Cu 5-1 3.0 4.0 1.5 5.5 10.0 1.5 2.94 4.01 1.58 10.35.57 1.47 5-2 3.0 4.0 1.5 5.5 11.5 1.5 3.04 3.96 1.57 11.7 5.66 1.44 5-33.0 4.0 1.5 7.5 10.0 1.5 2.98 3.95 1.55 10.2 7.57 1.45 5-4 3.0 4.0 1.57.5 11.5 1.5 3.01 3.95 1.56 11.8 7.52 1.45 5-5 3.0 4.0 2.5 5.5 10.0 1.53.01 3.94 2.66 10.3 5.62 1.48 5-6 3.0 4.0 2.5 5.5 11.5 1.5 2.95 4.042.63 11.8 5.57 1.47 5-7 3.0 4.0 2.5 7.5 10.0 1.5 3.03 3.97 2.65 10.07.49 1.46 5-8 3.0 4.0 2.5 7.5 11.5 1.5 3.03 3.99 2.61 11.1 7.46 1.42 5-93.0 4.0 1.5 5.5 10.0 0.0 2.91 3.93 1.59 9.9 5.63 0.036 5-10 3.0 4.0 1.55.5 11.5 0.0 2.92 3.92 1.58 11.4 5.58 0.017 5-11 3.0 4.0 1.5 7.5 8.5 1.53.00 3.92 1.58 8.4 7.47 1.43 5-12 3.0 4.0 1.5 7.5 8.5 0.0 3.01 3.94 1.68.4 7.63 0.036 R3 2.0 5.0 2.0 0.6 35.0 — 2.0 4.7 2.0 35.0 0.6 — R4 2.82.8 2.0 0.8 20.5 — 2.3 2.8 2.0 21.2 1.2 — R5 3.8 2.8 — 0.8 — — 3.5 2.7 —— 0.4 — or less R6 3.1 4.2 — 0.6 — Mo 2.8 4.4 — — 0.4 Mo or 0.5 0.47less Analyzed Composition of Entire Cast Product T.S. (% by mass)Equivalent (%) A.P. No. P Mg Ce S C_(eq) Ni_(eq) Cr_(eq) (%) *1 Remarks5-1 0.025 0.038 0.022 0.005 4.3 15.5 7.6 100 5-2 0.021 0.047 0.023 0.0054.4 16.9 7.5 100 5-3 0.03 0.040 0.023 0.005 4.3 16.3 7.5 100 5-4 0.0280.048 0.024 0.006 4.3 17.9 7.5 100 5-5 0.024 0.042 0.021 0.005 4.3 15.58.6 100 5-6 0.025 0.050 0.022 0.005 4.3 17.0 8.7 100 5-7 0.026 0.0410.020 0.005 4.4 16.1 8.6 100 5-8 0.025 0.046 0.022 0.004 4.4 17.2 8.6100 5-9 0.023 0.042 0.021 0.005 4.2 13.7 7.5 100 5-10 0.024 0.043 0.0210.005 4.2 15.1 7.5 100 5-11 0.022 0.046 0.022 0.005 4.3 14.5 7.5 1005-12 0.029 0.044 0.022 0.005 4.3 13.2 7.5 100 R3 0.017 0.066 — 0.010 3.536.3 8.9 100 JIS: FCDA-NiSiCr3522 (or ASTM: D-5S) R4 0.022 0.052 — 0.0183.3 22.8 6.1 100 JIS: FCDA-NiCr202 (or ASTM: D-2) R5 0.040 0.028 — 0.0104.4 1.2 4.1 0 JIS: FCD450 R6 0.038 0.036 — 0.009 4.2 1.2 6.7 0 HiSiMoFCD (Note) *1: “A.P.” stands for Austenite Proportion.

TABLE 5B Cast Structure Number of Spheroidized Particles Proportion (%)(pieces/mm²) (Graphite's (Graphite's Heat-resistant Strength Min.Particle Min. Particle Hardness (800° C.) Dia.: 5 μm) Dia.: 5 μm) Hv (20kgf) Proof Tensile T.S. Thickness of Test Specimen at Collected Sections(mm) Stress Stress Elongation No. 25 12 5 3 25 12 5 3 25 12 5 3 MPa MPa% 5-1 81 78 84 85 200 273 508 664 191 207 230 275 73 122 16 5-2 84 81 8580 176 279 735 1026 188 196 221 250 72 117 16 5-3 78 86 89 86 157 269430 636 211 221 253 339 78 132 16 5-4 79 83 88 86 269 329 635 997 198212 239 280 77 128 15 5-5 81 80 86 87 236 265 399 494 219 228 269 335 76136 18 5-6 77 84 89 85 168 265 424 623 220 222 247 302 75 133 19 5-7 7780 85 87 153 193 300 230 239 255 310 433 79 145 15 5-8 75 76 81 82 146245 343 366 226 233 269 343 77 140 18 5-9 74 81 84 77 165 275 481 253201 221 256 356 77 131 29 5-10 76 76 85 81 216 260 489 670 192 203 227302 75 128 29 5-11 77 78 83 84 201 360 441 396 219 233 275 403 76 130 165-12 74 79 86 87 172 266 199 145 228 267 424 548 79 144 26 R3 87 90 7566 177 454 859 1181 159 158 157 164 73 116 31 R4 Unmeasured 87 134 25 R5Unmeasured 27 48 35 R6 80 81 85 85 154 266 434 603 237 231 242 297 28 4372 Charpy Heat-resistant Impact Strength Value (800° C.) 0.R. Test *1 atR.T. Reduction Young's (800° C. × 100 hr.) with T.S. of Area ModulusO.W.I. *2 O.W.R. *3 V-notch No. % GPa mg/cm² mg/cm² J/cm² Remarks 5-1 1467 40 69 10.2 5-2 16 66 38 58 9.9 5-3 16 65 41 62 4.8 5-4 14 68 38 566.5 5-5 16 73 39 49 4.4 5-6 17 76 35 46 4.6 5-7 13 67 36 48 2.8 5-8 1665 33 41 3 5-9 26 63 48 76 7.7 5-10 27 67 41 65 9.5 5-11 14 63 42 81 5.35-12 23 68 50 94 3.8 R3 32 101 13 22 11.5 JIS: FCDA-NiSiCr3552 (or ASTM:D-5S) R4 23 90 45 66 24 JIS: FCDA-NiCr202 (or ASTM: D2) R5 35 41 2 832.9 JIS: FCD450 R6 68 65 5 37 1.2 HiSiMo FCD (or TSFCDA4) (Note) *1:“O.R. Test” stands for Oxidation Resistance Test. *2: “O.W.I.” standsfor Oxidized Weight Increment. *2: “O.W.R” stands for Oxidized WeightReduction.

TABLE 6A Blended Composition Analyzed Composition of of Basic ElementsEntire Cast Product T.S. (%) (% by mass) No. C Si Cr Mn Ni Cu (Mo) C SiCr Ni Mn Cu P Mg Ce S 6-1 3.0 4.0 2.5 5.5 10.0 1.5 3.03 3.90 2.50 10.005.40 1.50 0.03 0.05 0.02 0.01 6-2 3.0 4.0 2.5 5.5 10.0 0.0 2.95 4.002.50 10.00 5.40 0.02 0.03 0.05 0.03 0.01 6-3 3.0 4.0 2.5 5.5 13.0 1.52.97 3.90 2.50 13.00 5.40 1.40 0.03 0.05 0.03 0.01 6-4 3.0 4.0 2.5 5.58.0 1.5 2.94 4.00 2.60 8.10 5.50 1.50 0.03 0.06 0.04 0.01 6-5 3.0 4.01.5 5.5 10.0 1.5 2.99 4.00 1.50 9.90 5.50 1.50 0.03 0.05 0.03 0.01 6-63.0 4.0 1.5 5.5 13.0 1.5 3.00 4.00 1.50 13.10 5.40 1.50 0.04 0.06 0.030.03 R3 2.0 5.0 2.0 0.6 35.0 — 2.0 4.7 2.0 35.0 0.6 — 0.017 0.066 —0.010 R4 2.8 2.8 2.0 0.8 20.5 — 2.3 2.8 2.0 21.2 1.2 — 0.022 0.052 —0.018 R5 3.8 2.8 — 0.8 — — 3.5 2.7 — — 0.4 — 0.040 0.028 — 0.010 or lessR6 3.1 4.2 — 0.6 — Mo 2.8 4.4 — — 0.4 Mo 0.038 0.036 — 0.009 or 0.5 0.47less Analyzed Composition of Entire Cast Product (% by mass) ppm T.S. n= 1 n = 2 n = 1 n = 2 Equivalent (%) A.P. *1 No. O N C_(eq) Ni_(eq)Cr_(eq) (%) Remarks 6-1 8 5 66 65 4.3 15.1 8.4 100 6-2 2 5 81 80 4.313.6 8.5 100 6-3 2 2 79 79 4.3 18.0 8.4 100 6-4 3 4 98 98 4.3 13.3 8.6100 6-5 4 2 60 59 4.3 15.1 7.5 100 6-6 4 3 52 52 4.3 18.2 7.5 100 R3 — —— — 3.5 36.3 8.9 100 JIS: FCDA-NiSiCr3552 (or ASTM: D-5S) R4 — — — — 3.322.8 6.1 100 JIS: FCDA-NiCr202 (or ASTM: D-2) R5 — — — — 4.4 1.2 4.1 0JIS: FCD450 R6 — — — — 4.2 1.2 6.7 0 HiSiMo FCD (or TSFCDA4) (Note) *1:“A.P.” stands for Austenite Proportion.

TABLE 6B Cast Structure Number of Spheroidized Particles OxidationProportion (%) (pieces/mm²) Resistance (Graphite's (Graphite's (800° C.× 100 hr.) Heat-resistant Strength Min. Particle Min. Particle HardnessOxidized Oxidized (800° C.) Dia.: 5 μm) Dia.: 5 μm) Hv (20 kgf) WeightWeight Proof Tensile Elonga- T.S. Thickness of Test Specimen atCollected Pars (mm) Increment Decrement Stress Strength tion No. 25 12 53 25 12 5 3 25 12 5 3 (mg/cm²) (mg/cm²) (MPa) (MPa) (%) Remarks 6-1 8484 86 87 135 212 375 464 226 238 253 303 27 39 73 129 16 6-2 70 75 83 84109 157 184 191 235 267 270 387 77 46 77 140 25 6-3 81 82 83 81 160 209329 695 205 220 232 252 20 32 74 128 18 6-4 81 82 85 83 110 179 164 65250 280 350 523 23 49 77 132 20 6-5 78 81 85 86 212 225 404 762 205 223231 252 29 55 70 117 18 6-6 84 85 88 88 167 293 455 950 184 205 208 22422 44 70 115 17 R3 87 90 75 66 177 454 859 1181 159 158 157 164 13 22 73116 31 JIS: FCDA- NiSiCr3552 (or ASTM: D-5S) R4 Unmeasured 45 66 87 13425 JIS: FCDA- NiCr202 (or ASTM: D2) R5 Unmeasured 2 83 27 48 35 JIS:FCD450 R6 80 81 85 85 154 266 434 603 237 231 242 297 5 37 28 43 72HiSiMo FCD (or TSFCDA4)

TABLE 6C Oxidation Resistance Test (in air for 100 hours) Test OxidizedWeight Oxidized Weight Tensile Test Specimen Increment (mg/cm²)Decrement (mg/cm²) 0.2% Proof Stress (MPa) No. 750° C. 800° C. 850° C.750° C. 800° C. 850° C. R.T. 750° C. 800° C. 850° C. 6-1 28 27 24 22 3982 316 100 73 55 6-2 32 77 25 27 46 104 326 100 77 59 6-3 16 20 26 20 3240 292 100 74 55 6-4 15 23 24 25 49 102 334 106 77 54 6-5 21 29 25 33 55123 286 96 70 53 6-6 18 22 30 29 44 87 265 93 70 52 R3 8 6 6 12 14 12257 — 73 — R4 17 27 45 35 54 90 223 94 72 60 R5 17 27 45 104 171 309 416— 27 — R6 6 5 1 13 14 39 578 — 28 — Test Tensile Test Specimen TensileStrength (MPa) Elongation (%) No. R.T. 750° C. 800° C. 850° C. R.T. 750°C. 800° C. 850° C. 6-1 477 174 129 96 7 12 16 17 6-2 460 179 140 104 426 25 31 6-3 454 171 128 96 10 16 18 17 6-4 479 177 132 98 4 11 20 186-5 475 156 117 89 13 15 18 16 6-6 463 152 115 86 17 19 17 19 R3 463 —116 — 26 — 31 — R4 415 147 113 92 12 25 29 23 R5 546 — 48 — 7 — 35 — R6656 — 43 — 6 — 72 — Thermal-fatigue Test 200° C. <---> 800° C. 100%Constrained Ratio 50% Constrained Ratio Test Stress Stress Stress StressSpecimen Decline Stress Decline Stress Decline Fractured Decline DeclineDecline Fractured No. by 10% by 25% by 50% Apart by 10% by 25% by 50%Apart 6-1 77 — — 77 259 — — 259 6-2 — — — 58 245 257 274 318 6-3 55  5759 64 — — — 367 6-4 — — — 31 — — — 221 6-5 — — — 80 364 371 — 372 6-6 —— — 99 251 255 — 256 R3 — — — — — — — — R4 188  191 193  193  398 441508 565 R5 — — — — — — — — R6 — — — — — — — — Thermal-fatigue Test 200°C. <---> 800° C. 150° C. <---> 800° C. Linear 30% Constrained Ratio 100%Constrained Ratio Expansion Test Stress Stress Stress Stress StressCoefficient Specimen Decline Decline Decline Fractured Decline DeclineFractured (×10⁻⁶/K) No. by 10% by 25% by 50% Apart by 10% by 25% Apart(40-900° C.) 6-1  802  830  863  933 — — — 21 6-2 — — — — — — — — 6-3 —— — — — — — — 6-4 — — — — — — — — 6-5 1118 1146 1163 1211 — — — 20 6-61042 1059 1076 1087 108 110 116 20 R3 — — — — — — 117 17 R4 1346 13731402 1508 105 — 107 19 R5 — — — —  38  48 58 16 R6 — — — — — — 76 16

1. An austenitic cast iron, being characterized in that: it comprises:basic elements comprising carbon (C), silicon (Si), chromium (Cr),nickel (Ni), manganese (Mn) and copper (Cu); and the balance comprisingiron (Fe), inevitable impurities and/or a trace-amount modifier element,which is effective in improving characteristic, in a trace amount; it isan austenitic cast iron being a cast iron that is structured by a basecomprising an Fe alloy in which an austenite phase makes a major phasein ordinary-temperature region; wherein said basic elements fall withincompositional ranges that satisfy the following conditions when theentirety of said cast iron is taken as 100% by mass (hereinafter beingsimply expressed as “%”) : C : from 1 to 5%; Si: from 2 to 6%; Ni: from7 to 15% ; Mn: from 0.1 to 8%; Cu: 2.5% or less; Cr: 6% or less; andCu+Cr: 0.5% or more.
 2. The austenitic cast iron as set forth in claim1, wherein said Ni is from 8 to 12%.
 3. The austenitic cast iron as setforth in claim 1, wherein said Si is from 3 to 5%.
 4. The austeniticcast iron as set forth in claim 1, wherein said Mn is from 5 to 8%. 5.The austenitic cast iron as set forth in claim 1, wherein said Cr isfrom 0.5 to 4%.
 6. The austenitic cast iron as set forth in claim 5,wherein said Cr is from 1 to 2% .
 7. The austenitic cast iron as setforth in claim 1, wherein said Cu is 0.1% or more.
 8. The austeniticcast iron as set forth in claim 7, wherein said Cu is 0.5% or more. 9.The austenitic cast iron as set forth in claim 8, wherein said Cu isfrom 1 to 2%.
 10. The austenitic cast iron as set forth in claim 1,wherein: said Cr is 0.1% or more; and said Cu is 0.1% or more.
 11. Theaustenitic cast iron as set forth in claim 10, wherein: said Cr is 0.5%or more; and said Cu is 0.5% or more.
 12. The austenitic cast iron asset forth in claim 1, wherein: a value, Creq (i. e., Creq=Cr+1.5Si), isfurther from 5 to 8%; and another value, Ni_(eq) (i.e., Ni_(eq)=Ni+30″C_(s)+0.5″ Mn+Cu where C_(s): solute carbon content) is further 18% ormore.
 13. The austenitic cast iron as set forth in claim 1, wherein: aCr_(eq) value is from 7 to 9%; and a Ni_(eq) value is 13% or more. 14.The austenitic cast iron as set forth in claim 1, wherein said basicelements further fall within compositional ranges that satisfy thefollowing conditions: C: from 2.5 to 3.5% ; Si: from 3.5 to 5.5% ; Ni:from 9 to 14% ; Mn: from 1 to 6% ; Cr: from 1 to 2% ; and Cu: from 1 to2% .
 15. The austenitic cast iron as set forth in claim 1, wherein saidbasic elements further fall within compositional ranges that satisfy thefollowing conditions: C: from 2.5 to 3.5% ; Si: from 3.5 to 4.5%; Ni:from 12 to 14% ; Mn: from 5 to 6% ; Cr: from 1 to 2% ; and Cu: from 1 to2%.
 16. The austenitic cast iron as set forth in claim 1, wherein aspheroidized proportion of said graphite that is crystallized orprecipitated in said base is 70% or more.
 17. The austenitic cast ironas set forth claim 1, wherein said graphite that is crystallized orprecipitated has particles in a quantity of 100 pieces/mm² or more,particles whose particle diameter is 5 μm or more and which are presentin a section of cast product whose wall thickness is 5 mm or less. 18.The austenitic cast iron as set forth in claim 1, wherein said basecomprises an austenite single phase.
 19. A manufacturing process foraustenitic cast product, the manufacturing process being characterizedin that it comprises: a molten-metal preparation step of preparing amolten metal with the compositional range as set forth in claim 1; apouring step of pouring the molten metal into a casting die; and asolidification step of cooling the molten metal that has been pouredinto the casting die, and then solidifying the mol ten metal; wherein acast product comprising an the austenitic cast iron is obtainable, theaustenitic cast iron being made of a cast iron that is structured by abase comprising an Fe alloy in which an austenite phase makes a majorphase in oridinary-temperature region.
 20. A manufacturing process foraustenitic cast product, the manufacturing process being characterizedin that it comprises: a modifier-free-molten-metal preparation step ofpreparing a modifier-free molten metal comprising a molten metal withthe compositional range as set forth in claim 1; an auxiliary-agentaddition step of adding an auxiliary agent, which includes at least onemember being selected from the group consisting of inoculant agents thatmake cores of graphite to be crystallized or precipitated, andspheroidizing agents that facilitates spheroidizing of the graphite, tothe modifier-free molten metal directly or indirectly; a pouring step ofpouring a molten metal into a casting die, the mol ten metal being afterthe auxiliary-agent addition step or during the auxiliary-agent additionstep; and a solidification step of cooling the mol ten metal that hasbeen poured into the casting die, and then solidifying the molten metal;wherein a cast product comprising an austenitic cast iron is obtainable,the austenitic cast iron being made of a cast iron that is structured bya base comprising an Fe alloy in which an austenite phase makes a majorphase in ordinary-temperature region, and the in which substantiallyspheroidal graphite is crystallized or precipitated within the base. 21.An austenitic cast product being characterized in that the austeniticcast product is obtainable by means of the manufacturing process as setforth in claim
 19. 22. A component part for exhaust system beingcharacterized in that the exhaust-system component part is obtainable bymeans of the manufacturing process as set forth in claim
 19. 23. Anaustenitic cast product being characterized in that the austenitic castproduct is obtainable by means of the manufacturing process as set forthin claim
 20. 24. A component part for exhaust system being characterizedin that the exhaust-system component part is obtainable by means of themanufacturing process as set forth in claim 20.